EXPERIMENTAL INVESTIGATION OF AERODYNAMICS AND COMBUSTION PROPERTIES OF A MULTIPLE-SWIRLER ARRAY

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2 EXPERIMENTAL INVESTIGATION OF AERODYNAMICS AND COMBUSTION PROPERTIES OF A MULTIPLE-SWIRLER ARRAY A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of DOCTORATE OF PHILOSOPHY (Ph.D.) in the Department of Aerospace Engineering and Engineering Mechanics of the College of Engineering and Applied Science 214 by Yi-Huan Kao B.S., Natioal Tsing Hua University, 27 M.S., University of Cincinnati, 212 Committee Chair: Dr. San-Mou Jeng I

3 Abstract An annular combustor is one of the popular configurations of a modern gas turbine combustor. Since the swirlers are arranged as side-by-side in an annular combustor, the swirling flow interaction should be considered for the design of an annular gas turbine combustor. The focus of this dissertation is to investigate the aerodynamics and the combustion of a multiple-swirler array which features the swirling flow interaction. A coaxial counter-rotating radial-radial swirler was used in this work. The effects of confinement and dome recession on the flow field of a single swirler were conducted for understanding the aerodynamic characteristic of this swirler. The flow pattern generated by single swirler, 3-swirler array, and 5-swirler array were evaluated. As a result, the 5-swirler array was utilized in the remaining of this work. The effects of inter-swirler spacing, alignment of swirler, end wall distance, and the presence of confinement on the flow field generated by a 5-swirler array were investigated. A benchmark of aerodynamics performance was established. A phenomenological description was proposed to explain the periodically non-uniform flow pattern of a 5-swirler array. The non-reacting spray distribution measurements were following for understanding the effect of swirling flow interaction on the spray distribution issued out by a 5-swirler array. The spray distribution from a single swirler/ fuel nozzle was measured and treated as a reference. The spray distribution from a 5-swriler array was periodically non-uniform and somehow similar to what observed in the aerodynamic result. The inter-swirler spacing altered not only the topology of aerodynamics but also the flame shape of a 5-swirler array. As a result, the distribution of flame shape strongly depends on the inter-swirler spacing. i

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5 Table of Contents ABSTRACT TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES LIST OF SYMBOLS I II V XI XII CHAPTER 1: INTRODUCTION Introduction of Gas Turbine Combustor and Swirling Flow Literature Review and Motivation Scope and Objective of this Dissertation... 8 CHAPTER 2: EXPERIMENTAL SETUP AND SWIRLER Counter-Rotating, Radial-Radial Swirler Simplex Fuel Nozzle Test Facility Stokes Number and Validation of LDV Measurement CHAPTER 3: EFFECT OF CONFINEMENT ON THE FLOW FIELD GENERATED BY A COUNTER-ROTATING RAD-RAD SWIRLER Test Conditions Results and Discussions Conclusions CHAPTER 4: EFFECT OF DOME RECESSION ON THE FLOW FIELD GENERATED BY A COUNTER-ROTATING RAD-RAD SWIRLER Test Conditions ii

6 4.2 Results and Discussions Conclusions CHAPTER 5: EFFECT OF NUMBER OF SWIRLERS ON THE FLOW FIELD GENERATED BY A MULTIPLE-SWIRLER ARRAY Test Conditions Results and Discussions Conclusions... 5 CHAPTER 6: EFFECT OF INTER-SWIRLER SPACING ON THE FLOW FIELD GENERATED BY A 5-SWIRLER ARRAY Test Conditions Results and Discussions Conclusions CHAPTER 7: EFFECT OF ALIGNMENT OF SWIRLERS ON THE FLOW FIELD GENERATED BY A 5-SWIRLER ARRAY Test Conditions Results and Discussions Conclusions CHAPTER 8: EFFECTS OF END WALL DISTANCE AND CONFINEMENT ON THE FLOW FIELD GENERATED BY A 5-SWIRLER ARRAY Test Conditions Results and Discussions Conclusions iii

7 CHAPTER 9: PHENOMENOLOGICAL DESCRIPTION OF THE PERIODIC BEHAVIOR OF A 5- SWIRLER ARRAY Periodic Behavior among 5-swirler Array Switching the Periodic Mode CHAPTER 1: SPRAY DISTRIBUTION OF A 5-SWIRLER ARRAY Test Conditions Results and Discussions Conclusions CHAPTER 11: FLAME SHAPE OF A 5-SWIRLER ARRAY Test Conditions Results and Discussions Conclusions SUMMARY 124 REFERENCES 125 iv

8 List of Figures Figure 1.1 a sector of typical gas turbine combustor... 9 Figure 1.2: dome geometry of combustor in CFM Figure 2.1: a rad-rad, counter-rotating swirler assembly Figure 2.2: effective area measurements for 5 swirlers Figure 2.3: simplex fuel nozzle Figure 2.4: flow number measurement for 5 fuel nozzles (water as working fluid) Figure 2.5: flow number measurement for 5 fuel nozzles (Jet-A as working fluid) Figure 2.6: schematic of test facility for non-reacting LDV measurement Figure 2.7: schematic of test facility for no-reacting PDPA measurement Figure 2.8: Combustion Chamber Figure 3.1: coordinate system (confinement effect) Figure 3.2: axial velocity and axial-radial velocity vector contours (confinement effect); (a) 2D x 2D, (b) 2.5D x 2.5D, (c) 3D x 3D, (d) unconfined Figure 3.3: radial velocity contours (confinement effect); (a) 2D x 2D, (b) 2.5D x 2.5D, (c) 3D x 3D, (d) unconfined Figure 3.4: tangential velocity contours (confinement effect); (a) 2D x 2D, (b) 2.5D x 2.5D, (c) 3D x 3D, (d) unconfined Figure 3.5: axial velocity along the Z-axis (confinement effect)... 3 Figure 3.6: velocity profiles at Z/D=.12 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 3.7: velocity profiles at Z/D=.39 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities v

9 Figure 3.8: velocity profiles at Z/D=.98 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 3.9: velocity profiles at Z/D=1.57 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 4.1: description of dome recession distance Figure 4.2: axial velocity and axial-radial vector contours for a single confined swirler; (a) D r =.3D. (b) D r =.13D Figure 4.3: velocity profiles at Z/D=.12, confined case (dome recession effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities... 4 Figure 4.4: velocity profiles at Z/D=.98, confined case (dome recession effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 4.5: axial velocity and axial-radial vector contours for a single unconfined swirler; (a) D r =.3D. (b) D r =.13D Figure 4.6: velocity profiles at Z/D=.12, unconfined case (dome recession effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 4.7: flow pattern sketches for an unconfined single swirler with difference dome recession: (a) (nearly) no recession; (b) further recession Figure 4.8: velocity profiles at Z/D=.98, unconfined case (dome recession effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 5.1: coordinate system (multi-swirler array) Figure 5.2: axial velocity and axial-radial velocity vector contours (no. of swirlers); (a) single swirler, (b) three swirlers, (c) five swirlers Figure 5.3: velocity profiles at Z/D=.12 (no. of swirlers), (a) axial velocities, (b) radial velocities vi

10 Figure 5.4: velocity and turbulent kinetic energy profiles at Z/D=.63 (no. of swirlers), (a) axial velocities, (b) radial velocities, (c) tangential velocities, (d) turbulent kinetic energy distributions Figure 5.5: velocity profiles at Z/D=1.77 (no. of swirlers), (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 5.6: velocity profiles at Z/D=3.94 (no. of swirlers), (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 6.1: descriptions of inter-swirler spacing (S) and end wall distance (D w ) Figure 6.2: axial velocity and axial-radial vector contours (spacing effect); (a) S=1.75, (b) S=2, (c) S=2.5, (d) S= Figure 6.3: velocity profiles at Z/D=.12 (spacing effect); (a) axial velocities, (b) radial velocities Figure 6.4: velocity profiles at Z/D=.63 (spacing effect), (a) axial velocities, (b) radial velocities, (c) tangential velocities, (d) turbulent kinetic energy distributions Figure 6.5: velocity profiles at Z/D=1.77 (spacing effect), (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 6.6: velocity profiles at Z/D=3.94 (spacing effect), (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 7.1: coordinate system (multiple-swirler array, modified configuration with 1/8D offset from the center swirler) Figure 7.2: axial velocity and axial-radial vector contours (alignment effect); (a) baseline (b) 1/8D offset (modified) Figure 7.3: velocity profiles at Z/D=.2 (alignment effect), (a) axial velocities, (b) radial velocities Figure 7.4: velocity and turbulent kinetic energy profiles at Z/D=.63 (alignment effect), (a) axial velocities, (b) radial velocities (c) tangential velocities (d) turbulent kinetic energy vii

11 Figure 7.5: velocity profiles at Z/D=1.77 (alignment effect), (a) axial velocities, (b) radial velocities (c) tangential velocities Figure 7.6: velocity profiles at Z/D=3.94 (alignment effect), (a) axial velocities, (b) radial velocities (c) tangential velocities... 8 Figure 8.1: Axial velocity and axial-radial vector contours; (a) Dw=.75D, (b) Dw=1D, (c) Dw=1.25D, (d) Dw=2D, (e) Dw=unconfined Figure 8.2: velocity profiles, Z/D=.12 (end wall distance); (a) axial velocities, (b) radial velocities Figure 8.3: velocity profiles, Z/D=.12 (confinement effect); (a) axial velocities, (b) radial velocities.. 89 Figure 8.4: velocity and turbulent kinetic energy profiles, Z/D=.63 (end wall distance); (a) axial velocities, (b) radial velocities, (c) tangential velocities, (d) turbulent kinetic energy distributions Figure 8.5: velocity and turbulent kinetic energy profiles, Z/D=.63 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities, (d) turbulent kinetic energy distributions Figure 8.6: velocity profiles, Z/D = 3.94 (end wall distance); (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 8.7: velocity profiles, Z/D = 3.94 (end wall distance); (a) axial velocities, (b) radial velocities, (c) tangential velocities Figure 9.1: classical flow pattern of an array of five identical swirlers with high swirl number Figure 9.2: sketch of an alternating order of CTRZ distribution Figure 9.3: two possible periodic flow patterns of a 5-swirler array: (a) mode triggered by the center swirler (high D w /S); (b) mode triggered by the corner swirler (low D w /S) Figure 9.4: an unlikely flow pattern of a 5-swirler array with low D w /S Figure 9.5: the flow pattern with an offset center swirler viii

12 Figure 1.1: axial velocity contours of single swirler configuration: (a) gas phase, (b) liquid phase (~1μm), (c) liquid phase (~2μm), (d) liquid phase (2~6μm), (e) liquid phase (6~1μm) Figure 1.2: velocity profiles of single swirler, Z/D =.12; (a) axial velocities, (b) radial velocities.. 19 Figure 1.3: velocity profiles of single swirler, Z/D =.51; (a) axial velocities, (b) radial velocities.. 19 Figure 1.4: volume flux contour of single swirler Figure 1.5: mean diameter (D 1 ) contour of single swirler Figure 1.6: sauter mean diameter (D 32 ) contour of single swirler Figure 1.7: D 1 and D 32 comparisons of single swirler; (a) Z/D =.12, (b) Z/D = Figure 1.8: axial velocity contours of 5-swirler array: (a) gas phase, (b) liquid phase (~1μm), (c) liquid phase (~2μm), (d) liquid phase (2~6μm), (e) liquid phase (6~1μm) Figure 1.9: velocity profiles of 5-swirler array, Z/D =.12; (a) axial velocities (gas v.s. liquid), (b) radial velocities (gas v.s. liquid), (c) axial velocities (sorted liquid phase) Figure 1.1: velocity profiles of 5-swirler array, Z/D =.51; (a) axial velocities (gas v.s. liquid), (b) radial velocities (gas v.s. liquid), (c) axial velocities (sorted liquid phase) Figure 1.11: volume flux contour of 5-swirler array Figure 1.12: mean diameter (D 1 ) contour of 5-swirler array Figure 1.13: sauter mean diameter (D 32 ) contour of 5-swirler array Figure 1.14: D 1 and D 32 comparisons of single swirler; (a) Z/D =.12, (b) Z/D = Figure 1.15: spray distribution information of 5-swirler array at XY plane (Z/D =.51); (a) axial velocity contour, (b) volume flux contour, (c) mean diameter (D 1 ) contour, (d) sauter-mean diameter (D 32 ) contour Figure 11.1: flame shapes generated by a confined 5-swirler array with S = 2D among 8 test conditions ix

13 Figure 11.2: flame shapes generated by a confined 5-swirler array with S = 2.5D among 8 test conditions x

14 List of Tables Table 2.1: dimension parameters of swirler Table 2.2: details of LDV system Table 2.3: details of PDPA system xi

15 List of Symbols A eff CTRZ CRZ D D r D w D 1 D 32 d c d d dp FN G Z G θ LDV P PDPA R R o r effective area of swirler center torodial recirculation zone corner recirculation zone diameter of swirler dome recession distance end wall distance mean diameter of droplet sauter mean diameter of droplet characteristic dimension particle diameter pressure difference flow number axial flux of axial momentum axial flux of tangential momentum Laser Doppler Velocimetry mass flow rate of air total mass flow rate of fuel Pressure Phase Doppler Particle Analyzer gas constant radius of measurement domain radius location xii

16 rms S SN Stk TKE U U o Umean U rms Vmean V rms VF W Wmean W rms X Y Z ρ d root mean square inter-swirler spacing swirl number Stokes number turbulent kinetic energy axial velocity fluid velocity mean axial velocity rms of fluctuating axial velocity mean radial velocity rms of fluctuating radial velocity volume flux tangential velocity mean tangential velocity rms of fluctuating tangential velocity the location of X-axis of Cartesian coordinate system the location of Y-axis of Cartesian coordinate system the location of Z-axis of Cartesian coordinate system particle density µ g gas dynamics viscosity ɸ equivalence ratio xiii

17 Chapter 1: Introduction 1.1 Introduction of Gas Turbine Combustor and Swirling Flow A gas turbine is a type of internal combustion engine. The basic operating principle of a gas turbine is similar to that of a stream turbine, but fresh air is used instead of water (stream). The fresh air flows into a compressor section of a gas turbine and is brought to the high temperature and high pressure status. The energy will be added by injecting the fuel and going through a combustion process after that. These higher pressure and higher temperature air enters a turbine section and release the energy by expanding the volume of air and reducing the air temperature to an exhaust status. Under this procedure, the released energy will be converted to the shaft work which is used to drive the compressor and other device, such as electrical generator. In 1998, the gas turbine systems were accounted for around 15% of the power generation industry, and the demand is expected more likely exceeding percent as annual growth in the future. The market share of the gas turbine system in power generation system is believed will be up to 4 percent in 22 [1]. Fuel cell is still under the demonstration stage. Nuclear power plant is very expensive and needed passing through lots of hearings to solve some controversial issues. The efficiencies of wind power and solar power are still relatively low. They are also expensive and not available everywhere. Thus, the gas turbine system is still the most efficient and reliable power source and getting more and more important in our daily life. A gas turbine also is commonly used in a propulsion system, as turbojet, turbofan, or turboprop engine. As mentioned, the gas turbine combustor, where is fuel added, will affect the performance of a gas turbine system directly, in terms of efficiency, operating range, emission control and so on. Figure 1.1 shows a sector of typical gas turbine combustor, which is consisted of dome, inner liner, outer liner, fuel nozzle, and swirler. In order to achieve the performance target of a gas turbine combustor, the swirling flow is widely used. Not only in the gas turbine systems, the swirling flow, a highly complicated, three-dimensional flow structure in nature, is 1

18 also commonly used in the ramjets, and the industrial furnace burners. Typically, a swirling flow could be generated by introducing a swirl component to cause the pressure drop at the axis by a spinning flow. Therefore, a swirler assembly is in charge to generate a swirling flow in a gas turbine combustor. Usually, researchers evaluate the strength of swirling flow by a dimensionless number, swirl number (SN), as defined in Eq. (1-1). If the swirl number is greater than a critical number, around.5 ~.6, the adverse pressure gradient along the axis is enough to form a center toroidal recirculation zone (CTRZ) [2]. Once the CTRZ is established, it anchors the flame, thus improving the flame stability over the operating range of a gas turbine system. The turbulent properties of a CTRZ also improve the combustion reactant/ product mixing, combustion efficiency, flame stability, and emission control. For a gas turbine combustor, a swirl number is suggested to be greater than.6 [3, 4, 5, 6]. Therefore, in the last few decades, many researchers spent large amounts of effort to understand the characteristics of a swirling flow or a CTRZ. Mongia et al. [7] summarized the research works about swirling flow done between the combustion designer and the research community, and he [8] also indicated that the more accurate analytical design tools of understanding swirling flow are needed for developing and validating the next-generation combustor design. Generally speaking, the extensive works have been done could be divided to two major research groups. The first group limits their research on the design of swirler, such as vane angle, vane shape, hub size, and so on. The second group focuses on the operating condition of swirling flow or boundary condition relative to the combustor geometry. Eq. (1-1) 1.2 Literature Review and Motivation As mentioned, the introduction of swirl has been shown to have a significant impact on the development of the combustion flow field. The introduction of swirl creates the CTRZ and the corner 2

19 recirculation zone (CRZ), and the sizes of CTRZ and CRZ depend on the swirler design and swirl strength. When the swirl strength increases, the size of CRZ or the length of reattachment point is shorter. The associated CTRZ becomes shorter and wider as well [9, 1, 11, 12, 13, 14]. The turbulent intensity inside the combustor has also been revealed to be increased significantly with the use of swirler at the inlet of the combustor due to the vortex breakdown. The result is favorable for combustion because of the improvement of mixing property [15, 16]. Kilik [17] stated that increasing the vane outlet angle increases the size of recirculation zone, and the curved vane swirler can generate a larger and stronger CTRZ with less pressure loss through a swirler, as compared to the corresponding flat vane swirler with a same vane outlet angle. Kilik [18] also mentioned that the size of a recirculation region and the pressure loss through swirler both increase with increasing the blockage ratio for a fixed mass flow rate for an axial swirler. Syred et al. [19] showed that by inserting certain design of bluff body into or near the exit of the axial swirler, the size of CTRZ can be increased while that of the central forward flow reduces. Thus, the overall combustion system performance can be improved. Wang et al. [2] stated that the modest change in the flare design can cause a significant change not only in the gas phase but also in the two phase flow fields. The wider flare produces a larger CTRZ featuring lower reverse flow velocity and wider droplet dispersion, as compared to the narrow flare. In McDonell s work [21], it has been found out that the misalignment of fuel nozzle in a swirler assembly has a modest effect on the gas phase aerodynamics and overall droplet sizes, but the fuel droplet distribution has a significant change. In order to improve the mixing property and turbulent level at the exit of swirler, a coaxial duelswirlers assembly was proposed and studied by some researchers. Kilik et al. [22], Gherman et al. [23], and Vu et al. [24] concluded that the counter-swirling coaxial swirler provides higher turbulent level than the co-swirling one. This resulted higher turbulent level is favorable for combustor design in terms 3

20 of reducing the length of CTRZ, improving mixing, causing better fuel atomization, and helping flame stability due to increasing the flame speed. But in the reacting condition, a quench phenomenon observed in the shear layer between the inner and outer flows because of excessive turbulence should be avoided. In Panduranga Reddy s work [25, 26], he showed that the length and the width of CTRZ are decreased with decreasing the pressure drop (decreasing the Reynolds number) across the swirler in the confined combustor geometry. Fu et al. [27] did a similar study to investigate the effect of Reynolds number on the unconfined swirling flow structure. The result showed that the normalized swirling flow structure is highly similar with different Reynolds number. The additional conclusion in this work was that the existence of confinement has a significant impact on the swirling flow structure. The topic of confinement level upon a swirling flow has been widely studied by many researchers. Abujelala et al. [28] showed that not only CTRZ but also CRZ are strongly affected by the confinement level. The sizes of CRZ and CTRZ increase with increasing the size of confinement chamber, which was accord with the result obtained from Fu [29]. However, Fu s other work [3] revealed a different conclusion. There is only one CTRZ observed in a smaller confinement chamber, but two recirculation regions exist in a larger confinement chamber. This result suggested that there is a flow transition region happened at a certain confinement level. The discrepant conclusion among those research papers might be attributed to the different swirler, i.e. different swirler design and different swirl strength, utilized. Archer et al. [31] suggested that the confinement affects both non-burning and burning cases. The confinement promotes the chemical reactions and increases the turbulent levels under combustion conditions. 4

21 As mentioned before, Figure 1-1 shows a sector of typical gas turbine combustor. The effect of confinement level is associated with the size of width of combustor, i.e. the width between the inner liner and the outer liner. The other geometry parameters, such as dome geometry or exit contraction, have also been extensively investigated by other researchers. Rhode et al. [32, 33] stated that the CRZ can be eliminated by reducing the dome expansion angle which doesn t have a clear impact on the CTRZ. Mondal et al. [34] used k-ε model to indicate that the dome expansion angle modifies the flow pattern inside combustor only at the high swirl number and then near the inlet of the combustor. When the swirl number is very small, the flow pattern inside combustor is nearly unchanged. Kao et al. [35] got a similar result to Mondal s work by experimental method to show the dome expansion angle has a clear impact on the flow pattern in the inlet of combustor, in terms of eliminating the CRZ and changing the width of CTRZ by a coaxial rad-rad swirler with SN 1. The results from Yoon [36] and Lilley [37] suggested that the effect of downstream contraction depends on the swirl strength. The weak contraction of area ratio 2 has little effect on the weak and strong swirling flows, and a strong contraction of area ratio 4 has great effect on both the intermediate and strong swirling flows. Nickolaus s work [38] done by Large Eddy Simulation (LES) calculation was accord with the conclusions from Yoon and Lilley. Kao s [39] work showed that effect of chamber length is mainly observed in the near field region of swirler exit, in terms of turbulent kinetic energy and the expansion angle of swirling jet but doesn t has clear impact on the shape of CTRZ and the downstream flow field. Mohammad et al. [4, 41, 42, 43] concluded that the length of CTRZ inside a single annular combustor sector with a downstream contraction is reduced around 4%. If the swirling flow is utilized in a Rich-Burning/ Quick-Quench/ Lean-Burning (RQL) Combustor, the termination point (rear stagnation point) of CTRZ is defined by the primary dilution jets. In the reacting flow, the flame structure highly depends on the aerodynamics and local equivalence ratio. 5

22 Except the aerodynamic study, the investigation of spray characteristics is also very critical to understand and to estimate the combustion performance in a liquid-fueled gas turbine combustor. Wang s work [44, 45, 46] indicated that the swirl cup provides more uniform and finer droplets, as compare to the atomizer only, at the exit of swirl cup. The smaller droplet performs more closely than the larger droplet, and a significant slip velocity between gas phase and liquid phase exists. The prominent slip velocity suggests the strong momentum exchange happened between the phases. The author also proposed the magnitude of the droplet fluctuating velocity should be correlated with both the gas phase local velocity fluctuation and the droplet trajectory. Hence, it can explain the reason of the larger droplets having a greater fluctuation in axial velocity then the samller droplets in some cases. Ateshkati et al. [47] stated that the counter-swirling swirler increases the level of mixing that occurs in the downstream region of the swirler. The counter-swirling configuration swirler with venture can generate the smallest drop size distribution due to the intense shearing and the venture acting as a prefilming surface. Jeng et al. [48] examined the droplet distribution by using water and Jet-A as working fluids in a non-reacting flow. He found out that the Jet-A fuel is more susceptible in the atomization processes provided by the swirl cup in terms of droplet size and distribution due to the relatively lower surface tension and viscosity forces, as compared to water. Colby et al. [49] showed that the nonreacting droplets lose the feature of primary counter-swirling rapidly at downstream of venture lip, instead follow the secondary swirling. Whereas, in the reacting case, the reacting droplets show the primary counter-swirling as the volumetric expansion transports further downstream. Fu et al. [5] indicated the global spray structures are not affected significantly by fuel types or equivalence ratios in a reacting flow. The width of fuel leaner spray is wider, and the fuel spray becomes more compact when the fuel flow rate increases. 6

23 From the summary of spray characteristics, some differences between non-reacting flow and reacting flow are inferred to exist. Thus, some researchers worked on the comparison between the nonreacting flow and the reacting flow. Fu et al. [51] highlighted that the axial velocity increases and the length of CTRZ is shorter for the reacting case, as compared to the non-reacting flow. And there are two counter swirling velocity components observed in the reacting flow, whereas the non-reacting flow doesn t show any counter swirling component above the swirler exit. In Davoudzadeh s work [52], a gas-fueled, as methane was employed, his result was accord with Fu s result. A shorter CTRZ and high axial velocity are observed in the reacting flow case, as compared to the non-reacting flow case. Dugué stated that the effect of combustion reduced both the size and strength of the CTRZ. The combustion reduces the effect of centrifugal forces by increasing the flow inertia [53]. Most of these previous works have been done by a single swirler or a single swirling flow generator. However, the most popular aircraft engine, CFM56 is equipped on Boeing 737 series, and Airbus A32, A33, A34 series, and designed as an annular combustor configuration. The dome geometry of CFM 56 is shown in Figure 1.2. Those swirler/ fuel nozzle assemblies are arranged side-byside in the combustor. The nature of the swirling flow for such a case might be significantly different, as compared to a single swirler due to different boundary conditions. For example, the single swirler configuration does not feature the effect of interaction between the adjacent swirlers. The swirling flow interaction not only affects the swirling flow performance, but also plays an important role on the flame spreading process from one fuel nozzle/ swirler assembly to the next during initial ignition or the process of relight. Some of works considering the swirling flow interaction were toward the application in Lean Direct Injection (LDI) system in order to achieve the emission target [54, 55, 56, 57, 58, 59]. The literature for a dimensionally-arranged swirling array is still limited. 7

24 Kucukgokoglan et al. [6] showed the two counter-rotating burners having an asymmetrical decay of swirl by Particle Image Velocimetry (PIV) measurement. Krautkermer et al. [61] suggested that three co-rotating swirlers arranged linearly results in a better mixing efficiency, as compared to a counter-rotating configuration. Fanaca et al. [62] indicated that an annular multiple-burner combustor and a single burner combustor have significant difference on aerodynamic effect which results in the discrepancies in the flame transfer function. In particular, the high speed flow regime shifts from a wall jet flow in a single burner combustor to a free jet flow in an annular combustor. Cordier et al. [63] addressed that the ignition sequence strongly depends on the spacing of swirlers. A radial propagation mechanism is observed for a smaller spacing between swirlers. As the spacing increases, an axial propagation mechanism becomes dominant, and the time to achieve a full ignition also becomes very high. Due to the limited number of literature considering the swirling flow interaction available in public, in particular, for an annular gas turbine combustor, the benchmark of swirling flow interaction should be established to provide more helpful insight for developing the advanced gas turbine combustor in the future. 1.3 Scope and Objective of this Dissertation The aim of this dissertation is to investigate the aerodynamics performance and the droplet distribution of a linearly-arranged multiple-swirler array. Thus, the work began with studying the effects of confinement and dome recession on the aerodynamics of a single swirler to understand the inherent characteristics of a swirler used in this study. The second part compared the aerodynamics generated by a single swirler or a multiple-swirler array. As a result, a 5-swirler array was proposed to be utilized in the following sections. Thus, the effect of inter-swirler spacing, end wall distance, alignment, and 8

25 confinement on the aerodynamics of a 5-swirler array were investigated. The droplet distribution issued out by a single swirler/ fuel nozzle assembly was conducted in order to establish the benchmark for discussing the effect of swirling interaction on the droplet distribution of a 5-swirler array which was followed by examining the flame shape in a 5-swirler array with different inter-swirler spacing by taking high speed videos and pictures. Figure 1.1 a sector of typical gas turbine combustor Figure 1.2: dome geometry of combustor in CFM56 [61] 9

26 Chapter 2: Experimental Setup and Swirler A coaxial counter-rotating, radial-radial swirler assembly was used in this study. In non-reacting aerodynamics measurement, a two-component Laser Doppler Velocimetry (LDV) was used to measure the three mean velocity components and their corresponding rms values in order the obtain the mean flow structure and the turbulent properties. In non-reacting droplet distribution measurement, a twocomponent Phase Doppler Particle Analyzer (PDPA) was employed to measure the axial, radial velocities and their corresponding turbulent properties, as well as the droplet size and volume flux. A high speed camera was utilized to capture the flame shape in a 5-swirler array for the reacting cases. 2.1 Counter-Rotating, Radial-Radial Swirler The counter-rotating, radial-radial swirler has an inner, primary counter-clockwise rotating flow and an outer, secondary clockwise rotating flow, as seen from downstream toward swirler. The primary and the secondary air passage consist of 16 and 12 straight vanes, with vane angles of 53 o and 7 o, respectively, as shown in Figure 2.1 and listed in Table 2.1. The half diverging angle of flare is 54 o. The diameter of swirler exit, D, is 1 inch. The effective area of swirler is around.138 inch 2 at the pressure drop of 4% of 1 atm atmosphere across the swirler, which is a typical pressure drop setting of air in this study. However, there are five swirlers used in the multiple-swirler array study, Figure 2.2 shows the effective area measurement results of these five swirlers, which indicates these swirlers having very a similar effective area value within less than 8% discrepancy. When estimating the effective area of swirler, the mass flow rate was measured by CMF25 micro motion coriolis flow meter, and the pressure drop across swirler was monitored by Meriam 21 series smart pressure gauge. So the calculated effective area of swirler could be obtained by a simplified Eq. (2-1). 1

27 Eq. (2-1) The swirl number (SN) of this swirler was calculated by experiment data taken by a single swirler under 2D x 2D confined case which will be reported in Chapter 3. A simplified SN equation with neglecting the effects of pressure and density change is used for the calculation, as shown in Eq. (2-2), and the obtained SN of this swirler is around 1. Eq. (2-2) The presence of fuel nozzle was represented by a dummy fuel nozzle at the same location for the aerodynamics measurement by LDV. 2.2 Simplex Fuel Nozzle A typical simplex fuel nozzle was employed for the non-reacting droplet distribution measurement and reacting flow test, as shown in Figure 2.3. There were five simplex fuel nozzle used in the study measurement of a 5-swirler array. Those simplex fuel nozzles have spray angle of 75 o and flow number around.76~.8 with water as working fluid, and flow number around.81~.86 with Jet-A as working fluid at the upstream pressure of 5 psig, as shown in Figure 2.4 and 2.5. Due to the property difference between water and Jet-A, i.e. higher surface tension and stronger molecular bonding of water, the mass flow rate will be lower while water as working fluid with the same upstream pressure of nozzle. Thus, the flow number will be smaller when water is working fluid. 11

28 2.3 Test Facility A schematic of test facility for non-reacting LDV is shown in Figure 2.6. The air supply system consists of an Atlas Copco GA9-series compressor, rated for a flow rate of 512 cfm at a pressure of 15 psig, with a built-in demoisturizer system with a 37 o F dew point, a 14 gallon air settling tank, a pressure regulator and a series of 2 inch piping to connect it to the test stand. A Seeding generator capable of generating a nominal drop size of 5 microns olive oil droplets is used to seed the airflow to enable LDV measurements. The test setup consists of an upstream air manifold, the swirler adapter plate, and the test chamber which represents the enclosure for the swirler. The air manifold contains a honeycomb air straightener, a type-k thermocouple for air temperature monitoring, and a pressure port connected to a Meriam 21 series smart pressure gauge to measure the pressure drop across the swirler. A two-component, forward-scattering LDV system, PDI-2 series system from Artium Technologies Inc., was used to measure the velocity in the flow field. The LDV system consisted of a transmitter, a receiver, and advanced signal analyzer processors. The transmitter and the receiver were mounted on a computer-controlled 3-D traverse system with.1% accuracy of movement to scan the flow field. The reported time-averaged velocity data was typically based on more than 3, sample counts at each location with 5-second sampling period. The sample counts fall down to around 3, at the region closed to the wall of the test chamber. While measuring a swirling jet or a CTRZ, the sample counts up to 8, were observed. More details of LDV system are listed in Table 2.2. A schematic of test facility for non-reacting PDPA is shown in Figure 2.7. The first major test setup difference between LDV and PDPA measurements is that PDPA measurement was conducted horizontally to prevent the droplet mist accumulated inside the measurement domain. The same system, a two-component, forward-scattering PDPA system, PDI-2 series system from Artium Technologies Inc., was used to measure the droplet velocity and droplet size in the flow field. Due to the safety issue 12

29 of a long-period measurement, water was used as the working fluid for PDPA measurement. The water was stored in a stainless water tank and pressurized by the compressed air supply from the air compressor. The total mass flow rate of water was monitored by a CMF25 micro motion coriolis flow meter. The individual mass flow rate of water for each fuel nozzle was controlled and monitored by a FL-series rotameter from OMAGE Engineering Inc. and a PX429-series pressure transducer from OMAGE Engineering Inc. The reported time-averaged velocity and droplet size data was typically based on more than 3, sample counts at each location with 5-second sampling period. For the reacting tests, Jet-A was used as fuel. The method of flow rate control of Jet-A was identical to what had been used in non-reacting droplet measurement for controlling water flow rate. A Phantom v7.3 high speed camera from Vision Research Inc. is used to capture the flame shape in the burning conditions. The air is from the air tank and preheated by a 36 kw heater from BIG CHIEF Inc., before passing through the manifold and going to the test section. The combustion chamber employed for the reacting tests in shown in Figure 2.8. An 8.5 inch x 4 inch quartz window was installed on one of side wall for optical access, and a standard 2W electrical spark plug for a small aircraft was installed on the other side wall for the ignition purpose. The inner dimension of combustion chamber could be modified as 1 inch x 2 inch or 12 inch x 2 inch to investigate the flame shape in a 5-swirler array with different inter-swirler spacing of 2D or 2.5D, respectively, where D is the diameter of swirler exit. 2.4 Stokes Number and Validation of LDV Measurement In order to validate the gas phase flow field presented by the LDV measurement, the seeding particle velocity should be ensured to be representable for gas velocity. Stokes number (Stk), as defined in Eq. (2-3), a dimensionless number is used to evaluate the behavior of particles suspended in a flow 13

30 field. Stokes number presents as a ratio the characteristic time of a particle to the characteristic time of the flow. For Stk >> 1, the seeding particle is not able to attach a flow, especially where the high velocity gradient exists. For Stk << 1, the seeding particle follows the flow streamline closely. The tracing accuracy error is believed less than 1%, if Stk <<.1. Eq. (2-3) For this study, the typical absolute velocity magnitudes of a swirling jet and a CTRZ are around 2m/s and 5m/s, respectively. The corresponding calculated Stk values would be.47 and.12. It means the seeding particle follows the flow stream reasonably well, and the slip velocity between seeding particle and air stream could be neglected. 14

31 Figure 2.1: a rad-rad, counter-rotating swirler assembly effective area (in 2 ), Aeff SW#1 SW# SW#3 SW#4 SW# pressure drop (inh2) Figure 2.2: effective area measurements for 5 swirlers Figure 2.3: simplex fuel nozzle 15

32 water as working fluid nozzle #1 nozzle #2 nozzle #3 nozzle #4 nozzle #5 Flow Number Pressure Drop (psig) Figure 2.4: flow number measurement for 5 fuel nozzles (water as working fluid) Jet-A as working fluid.85 Flow Number nozzle #1 nozzle #2.725 nozzle #3 nozzle #4 nozzle # Pressure Drop (Psig) Figure 2.5: flow number measurement for 5 fuel nozzles (Jet-A as working fluid) 16

33 Figure 2.6: schematic of test facility for non-reacting LDV measurement Figure 2.7: schematic of test facility for no-reacting PDPA measurement 17

34 Figure 2.8: Combustion Chamber 18

35 Table 2.1: dimension parameters of swirler diameter of flare (swirler exit), D diverging angle of flare (half angle) 1 inch no. of vanes, primary air passage 16 vane angle, primary air passage no. of vanes, secondary air passage 12 vane angle, secondary air passage 54 o 53 o 7 o effective area.138 in 2 swirl number, SN 1 Table 2.2: details of LDV system Channel 1 (green) 2 (blue) wavelength (nm) focal length (mm) 5 5 fringe spacing (µm) beam diameter (mm) beam separation (mm) beam waist (μm) velocity measurement range -1 ~ +1 m/s velocity accuracy 1% Table 2.3: details of PDPA system Channel 1 (green) 2 (blue) wavelength focal length 1 1 fringe spacing beam diameter beam separation beam waist velocity measurement range -1 ~ +1 m/s velocity accuracy 1% droplet size measurement range ~ 1 μm droplet size accuracy 1μm 19

36 Chapter 3: Effect of Confinement on the Flow Field Generated by a Counter- Rotating Rad-Rad Swirler As discussed in Chapter 1, the confinement has a clear impact on the swirling flow, so the confinement can affect the aerodynamics, flame shape, heat transfer, operating range, lean blowout limitation, and combustion stability of a gas turbine combustor. Therefore, this chapter will focus on the effect of confinement on the swirling flow characteristic generated by this counter-rotating, radial-radial swirler. This is very important to understand the inherent characteristics of swirler which is used in this study. 3.1 Test Conditions Three different square ducts with different widths of 2D, 2.5D, and 3D were used in this work, where D is the diameter of swirler exit. The corresponding confinement ratios for these three confined levels are 5.1, 7.96, and 11.46, respectively. The confinement ratio is defined as a ratio of the cross section area of confinement to the swirler exit area. An additional unconfined case was examined as well. All of data were taken at the air pressure drop as 4±.1% of 1 atm atmosphere pressure between the upstream of the swirler and the downstream ambient. Air temperature ranged from 65 to 75 o F. The coordinate system is shown in Figure 1. The axis of swirler is designated as Z-axis, and the lateral axis perpendicular to Z-axis is assigned as X-axis. The origin of this coordinate system is at the center of the swirler exit plane. The measurements were taken along the X-axis with a.4d spatial resolution for the 2D x 2D, 2.5D x 2.5D, and the additional unconfined case. A.8D spatial resolution along the X-axis was only used in the 3D x 3D case while measuring. The measurement span covers around 9% of chamber width for the three confined cases. 2 and 12 different axial locations were measured for the three confined cases and the unconfined case, respectively. 2

37 3.2 Results and Discussions The Figure 3.2 shows the axial velocity and the axial-radial velocity vector contours for the three different confinement levels and an additional unconfined case. The relative length of vector is used in Figure 3.2. Figure 3.3 and Figure 3.4 show the radial and tangential velocity contours for the three cases, respectively. All of cases have a center torodial recirculation zone (CTRZ). For the three confined cases, the CTRZs are close-shaped bubbles, and the overall size increases with increasing of the chamber size. On the other hand, due to the unbounded reverse flow pattern, the boundary of CTRZ of the unconfined case is very hard to be determined. The highest reverse flow velocity is located at the core of CTRZ for the 2D x 2D case, as opposite to the swirler exit for the 3D x 3D and the unconfined cases. And the 2.5D x 2.5D case has strongest reverse velocities at the both locations. Figure 3.5 shows the axial velocity along Z-axis, the downstream location with zero axial velocity, called as rear stagnation point which is always used to determine the length of CTRZ. The length of CTRZ increases with increasing of the chamber size, except the unbounded CTRZ from the unconfined case. Figure 3.6 indicates the three velocity components at Z/D =.12, all of cases exhibit a reverse flow region near the axis of swirler. It means that the CTRZ extends inside the swirler, and the flow comes out from the swirler as a hollow cone structure accompanied with a high speed swirling jet at the outside. The 2.5D x 2.5D, 3D x 3D, and the unconfined cases all show strong negative axial velocity near the wall of chamber, which indicates the existence of CRZs clearly. The CRZ is barely measured in the measurement span of the 2D x 2D case because of the limitation of LDV measurement in the near wall region. But the existence of CRZ in the 2D x 2D case is still believed. Usually, the size of CRZ or the location of reattachment point depends on the step height or the difference between the half of chamber width and the swirler radius. For this swirler, the CRZ is very small for the 2D x 2D case, as compared to the other two confined cases. But the 2.5D x 2.5D and 3D x 3D cases have similar location 21

38 of reattachment point, at around Z/D =.5, and the similar size of CRZ. This indicates that the size of CRZ does not very strongly depend on the step height in the larger confinement cases, which is believed due to the high dispersing characteristic in the lateral direction of this swirler. The positive radial velocity at positive X location means the flow going away from the axis of swirler and the CTRZ expands its width. Due to the fast spreading of swirling jets issue out from the swirler, the high magnitude of radial velocity is expected at the exit of swirler. The wider radial velocity distribution and the stronger radial velocity magnitude are observed with increasing of the chamber size. Also, for the larger confined case, such as the 2.5D x2.5d, 3D x 3D, and the unconfined cases show a slight reverse trend of radial velocity at the region near the centerline. It is believed the shrinkage of size of the strong reverse flow at the exit of swirler. The positive tangential velocity at the positive X location means the flow rotating by a clockwise direction, as seen from downstream toward the swirler. The outer, secondary air passage generates a clockwise rotating flow, so the air flow from the secondary air passage dominates the flow structure for all cases basically. However, for the larger confined cases (2.5D x 2.5D and 3D x 3D) and the unconfined cases all show a slight reverse rotating direction, as a counter-clockwise rotation which is generated by the inner, primary air passage near the axis of swirler. This can conclude that the strong reverse flow at the exit of swirler is generated by the inner, primary air passage. For this coaxial counter-rotating, rad-rad swirler, the primary and secondary air passages are separated by a venturi. If the tangential momentum provided by the primary air passage is strong enough, a recirculation region could be formed in the region surrounded by venturi. After the venturi, the other CTRZ generated by the outer, secondary air passage will start to interact with the CTRZ generated by the inner, primary air passage and dominate it due to the higher swirl number, i.e. the stronger swirling 22

39 intensity, from the outer, secondary air passage. So the CTRZ of the inner, primary air passage will be merged into the CTRZ of the outer, secondary air passage. For the smaller confinement, the limited room forces these two swirling flows generated by the primary and the secondary air passages to have a stronger interaction and to merge together quickly, as compared to the larger confinement cases. Thus, the remaining CTRZ of the inner, primary air passage still could be observed in the larger confinement cases and the unconfined cases but does not exist in the highly confined 2D x 2D case. Figure 3.7 shows the three velocity components at Z/D =.39. For the 2D x 2D case, the swirling jet with the high axial velocity is already attaching on the wall, and the CTRZ almost reaches its maximum width at this axial position. However, for the 2.5D x 2.5D and 3D x 3D cases, the axial velocity profiles indicate that the high speed jets do not attach on the wall yet and the CTRZs are still expanding their sizes. The unconfined case shows a totally different axial velocity profile, as compared to those confined cases. The high speed swirling jet already diffuses away and cannot be observed at this location. A very wide CTRZ is established. For the 2D x 2D case, a fast decay of radial velocity is shown, the absolute value of peak radial velocity dropped from around 2 m/s at Z/D =.12 to around 1 m/s at Z/D =.39. This is attributed to the CTRZ reaching its maximum width, as mentioned before. Whereas, the absolute values of peak radial velocity for the 2.5D x2.5d and 3D x3d cases do not drop significantly. And the radial velocities for these two cases become wide-band distributions, which mean the swirling jets of these two configurations still having very high dispersing rate for expansion of CTRZ in the lateral direction. On contrary, the unconfined case shows a relatively flat distribution of the radial velocity and doesn t have any predominant radial velocity peak. The unconfined case also shows slight positive, negative radial velocities at negative, positive X locations at the center portion, respectively, as opposite to what 23

40 observed in the three confined cases. This might be attributed to the reducing size of the stronger reverse flow region near the axis of swirler. Similar to the radial velocity profile, the unconfined case has a relatively flat tangential velocity profiles due to the fast diffusion of swirling characteristic under an unconfined boundary condition. For the three confined cases, the strength of swirling flow increases with decreasing of the chamber size. Additionally, the 2D x 2D case rotates as a rigid-body rotation (forced vortex), but the other two cases perform like a Rankine vortex, which has a forced vortex near the axis of swirler and a free vortex at the outer region. Furthermore, the behavioral difference of tangential velocity between 2D x 2D case and the other two larger confined cases should be attributed to whether the high speed swirling jet reaches to the wall. Figure 3.8 shows the three velocity components at Z/D =.98, and the three velocity component profiles become very flat in the unconfined case which has a larger reverse flow pattern with reverse axial velocity value around 1.5 ~2 m/s and almost value of in the both radial and tangential velocity profiles. For the three confined cases, the swirling jet are attaching to the chamber wall, so the radial velocity profiles all approach to value of in all measurement span for three confined cases. And the CTRZs of all three confined cases reach their maximum width which is constrained by the size of chamber, so the maximum width of CTRZ increases with increasing the chamber size for this swirler could be drawn. The trend of tangential velocity intensity is the same with what observed at Z/D =.39, but the 2D x 2D case changes its tangential velocity profile from a rigid-body rotation to a Rankine vortex due to the effect of further wall friction. The 2.5D x 2.5D and 3D x 3D cases changes their tangential velocity profiles from a Rankine vortex to a rigid-body rotation, since the CTRZs in these two cases have the maximum width. 24

41 Figure 3.9 shows the three velocity components at Z/D = At this axial location, the radial velocity profiles of the 2.5D x 2.5D and 3D x 3D cases are still remaining to value of, so the CTRZs of these two cases do not start contracting the sizes. But the 2D x 2D case shows the slight negative and positive values of radial velocity at positive and negative X locations, respectively. This means the CTRZ of the 2D x 2D case is reducing its width, which can be also evident by the lower magnitude of negative axial velocity near the axis of swirler and the narrower negative axial velocity span. All of the tangential velocity profiles of the confined case all changed to a Rankine vortex should be noted. 3.3 Conclusions 1. A swirling flow structure is highly affected by the confinement level. 2. The swirler used in this research shows a highly dispersing characteristic in the lateral direction. 3. For the three investigated confined cases, a closed-shape CTRZ is observed. On contrary, due to the higher lateral dispersing rate of this swirler, the CTRZ is unbounded under an unconfined boundary condition. 4. For confined case, the size of CTRZ is constrained by the chamber size, so the overall size of CTRZ increases with increasing of the chamber size, in terms of width and length of CTRZ. And the magnitude of the reverse flow inside the core of CTRZ increases with increasing of the chamber size. 5. The size of CRZ and the reattachment length for confined case are not strongly depending on the step height due to the highly dispersing rate in the lateral direction of this swirler. 6. For the tightly confined case, the limited room forces two counter-rotating swirling flows to exchange momentums quickly and results in the CTRZ of the outer, secondary air passage to 25

42 dominate the flow structure right after the exit of swirler. On contrary, the remaining CTRZ of the inner, primary air passage is still observed at the exit of swirler for the larger confinement cases or the unconfined case. 26

43 Figure 3.1: coordinate system (confinement effect) (a) (b) (c) (d) Figure 3.2: axial velocity and axial-radial velocity vector contours (confinement effect); (a) 2D x 2D, (b) 2.5D x 2.5D, (c) 3D x 3D, (d) unconfined 27

44 (a) (b) (c) (d) Figure 3.3: radial velocity contours (confinement effect); (a) 2D x 2D, (b) 2.5D x 2.5D, (c) 3D x 3D, (d) unconfined 28

45 (a) (b) (c) (d) Figure 3.4: tangential velocity contours (confinement effect); (a) 2D x 2D, (b) 2.5D x 2.5D, (c) 3D x 3D, (d) unconfined 29

46 2 Axial Velocity (m/s) D x 2D D x 2.5D 3D x 3D unconfined Z/D Figure 3.5: axial velocity along the Z-axis (confinement effect) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D=.12 Axial Velocity (m/s) (a) Radial Velocity (m/s) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D= (b) 3

47 Tangential Velocity (m/s) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D= (c) Figure 3.6: velocity profiles at Z/D=.12 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D=.39 Axial Velocity (m/s) (a) Radial Velocity (m/s) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D= (b) 31

48 Tangential Velocity (m/s) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D= (c) Figure 3.7: velocity profiles at Z/D=.39 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D=.98 Axial Velocity (m/s) (a) Radial Velocity (m/s) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D= (b) 32

49 Tangential Velocity (m/s) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D= (c) Figure 3.8: velocity profiles at Z/D=.98 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D=1.57 Axial Velocity (m/s) (a) Radial Velocity (m/s) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D= (b) 33

50 Tangential Velocity (m/s) D x 2D 2.5D x 2.5D 3D x 3D unconfined Z/D= (c) Figure 3.9: velocity profiles at Z/D=1.57 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities 34

51 Chapter 4: Effect of Dome Recession on the Flow Field Generated by a Counter- Rotating Rad-Rad Swirler The aim of this chapter is to investigate the effect of dome recession on the flow field. The effect of dome recession was conducted with the confined and unconfined swirling flows. 4.1 Test Conditions All of data were taken at the air pressure drop as 4±.1% of 1 atm atmosphere pressure across the swirler. Air temperature ranged from 65 to 75 o F. Two different dome recession distance, as.3d and.13d were investigated. The description of dome recession distance is shown in Figure 4.1. For the confined case, 2D x 2D square chamber was used to be as a confinement. The coordinate systems used in this chapter are identical to the coordinate systems described in Chapter 3. The spatial resolution of LDV measurement is also the same with what mentioned in Chapter Results and Discussions Figure 4.2 shows the axial velocity and axial-radial vector contours for the confined case with two different dome recession distance (D r ), respectively. The relative length of vector is used in the Figure 4.2. For the confined conditions, the overall topology of the fluid mechanics for both cases looks similar. They both feature a compact CTRZ and the CRZs. However, there still exist several differences between the two cases. Figure 4.3 indicates the three velocity components at Z/D =.12 for two confined cases. The case with D r =.13D features higher magnitude of axial velocity of the swirling jet, as compared to the case with D r =.3D. Although the magnitude of radial velocity of the swirling jet is also higher in the case with D r =.13D. A larger CRZ and a smaller expansion angle of swirling jet are observed in the case with D r =.13D, as compared to that of D r =.3D. As a result, the reattachment point of the case with D r =.13D is located further downstream, around Z/D =.4. Whereas, the 35

52 reattachment point in the case with D r =.3D is at around Z/D =.2. Due to the existence of larger CRZ, the CRZ acts as an obstacle to prevent the swirling jet issued from the swirler to expand wider in the near field region of swirler exit. It causes the tangential velocity distribution is more concentrated along the swirler axis in the case with D r =.13D. Figure 4.4 shows the three velocity components at Z/D =.98. As the flow develops further downstream, the aerodynamic difference between the two confined cases with different dome recession distance is getting obscure. The case with D r =.13D has higher axial momentum along the wallattaching jet and higher reverse flow inside the CTRZ, which might be attributed to the swirling jet with high axial velocity from the exit of swirler. The radial and tangential velocity profiles between two cases are highly similar. The differences between two cases with different dome recession distance are more pronounced under the unconfined boundary condition. Figure 4.5 shows the axial velocity and axial-radial vector contours for the unconfined case with two different dome recession distances. For the case with D r =.3D, a large expansion angle of the swirling jet is observed and results in the swirling jet attach along the dome plate when the swirling flow is issuing out. An unbound CTRZ is obtained. On the contrary, a closed-shaped CTRZ bubble is observed when the dome recession distance increases to.13d. And the unconfined swirling flow in the case with D r =.13D also features a swirling jet with higher axial velocity and a stronger reverse flow inside the CTRZ. Therefore, a further recessed dome plate will cause swirling jet switching from a dome-attaching swirling jet to a free swirling jet could be concluded. Figure 4.6 indicates the three velocity components at Z/D =.12 for two unconfined cases. The case with D r =.13D shows two strong peaks on the axial velocity profile which match to those at the radial and tangential velocity profiles. On the other hand, the peaks of axial velocity in the case D r = 36

53 .3D is not as high as those in the case D r =.13D. The radial and tangential velocity profiles also display the wide-band distributions, indicating that the air flow issued from the swirler disperses away at the exit of swirler, for the case with D r =.3D. The case with D r =.3D has a stronger and wider reverse flow region at the outer region. In the case with D r =.13D, an outer reverse flow region is also observed next to the axial velocity peak, but the axial velocity turns to positive again further away from the swirler. After carefully observation, it is found that it coincides with a reversal in the radial velocity profile, indicating that an air stream from the ambient is coming towards the swirling jet by this location. Figure 4.7 shows flow pattern sketches for an unconfined single swirler with different dome recession. When the dome plate is further recessed (as D r =.13D in this study), the air passage size to supply entrained air to the swirling flow structure is adequate. Thus, the swirling jet performs as a free swirling jet due to the strength of low pressure region reduced. For the case with a smaller dome recession distance (as D r =.3D in this study), the air passage is not adequate to supply the entrained air for the high swirling flow structure. Therefore, a vacuum region is created near the dome plate causing the swirling jet to expand in the lateral direction and to attach to the dome plate. As the flow goes further downstream, the aerodynamic differences between the two cases persist. Figure 4.8 shows the three velocity components at Z/D =.98. There are two humps and a reverse flow region along swirler axis observed in the axial velocity profile in the case with D r =.13D, indicating that the CTRZ still exist by this location. The radial velocity profile shows that the size of CTRZ in the case with D r =.13D is shrinking, and the tangential momentum still remains high. But for the case with D r =.3D has relatively flat distributions on the three velocity components. It means the tangential momentum and the strength of swirling flow has been dispersed away by this location. 37

54 4.3 Conclusions 1. For the confined cases, the effect of dome recession is not very significant. Both cases (D r =.3D & D r =.13D) feature a compact CTRZ and CRZs. The further recessed dome results in the larger CRZs. As a result, the swirling jet expands narrower in the case with D r =.13D. But the effect of dome recession doesn t propagate to downstream clearly in the confined case. 2. The effect of dome recession has a dramatic influence on the flow pattern of unconfined cases. When the dome is nearly aligned with the trailing edge of swirler flare (D r =.3D), the swirling jet acts as a dome-attaching swirling jet. As the dome is recessed further, the swirling jet switches to a free swirling jet. 3. In the unconfined case with D r =.13D, a compact CTRZ is observed, and the tangential momentum and swirling strength are able to propagate to further downstream. 38

55 Figure 4.1: description of dome recession distance (a) (b) Figure 4.2: axial velocity and axial-radial vector contours for a single confined swirler; (a) D r =.3D. (b) D r =.13D 4 Z/D=.12 3 Axial Velocity (m/s) Dr=.3D Dr=.13D (a) 39

56 4 Z/D=.12 3 Radial Velocity (m/s) Dr=.3D Dr=.13D (b) 15 Z/D=.12 1 Tangential Velocity (m/s) Dr=.3D Dr=.13D (c) Figure 4.3: velocity profiles at Z/D=.12, confined case (dome recession effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities 4 Z/D=.98 3 Axial Velocity (m/s) Dr=.3D Dr=.13D (a) 4

57 4 Z/D=.98 3 Radial Velocity (m/s) Dr=.3D Dr=.13D (b) 15 Z/D=.98 1 Tangential Velocity (m/s) Dr=.3D Dr=.13D (c) Figure 4.4: velocity profiles at Z/D=.98, confined case (dome recession effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities 41

58 (a) (b) Figure 4.5: axial velocity and axial-radial vector contours for a single unconfined swirler; (a) D r =.3D. (b) D r =.13D 5 Z/D=.12 4 Axial Velocity (m/s) Dr=.3D Dr=.13D (a) 4 Z/D=.12 3 Radial Velocity (m/s) Dr=.3D Dr=.13D (b) 42

59 15 Z/D=.12 1 Tangential Velocity (m/s) Dr=.3D Dr=.13D (c) Figure 4.6: velocity profiles at Z/D=.12, unconfined case (dome recession effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities (a) (b) Figure 4.7: flow pattern sketches for an unconfined single swirler with difference dome recession: (a) (nearly) no recession; (b) further recession 5 Z/D=.98 4 Axial Velocity (m/s) Dr=.3D Dr=.13D (a) 43

60 4 Z/D=.98 3 Radial Velocity (m/s) Dr=.3D Dr=.13D (b) 15 Z/D=.98 1 Tangential Velocity (m/s) Dr=.3D Dr=.13D (c) Figure 4.8: velocity profiles at Z/D=.98, unconfined case (dome recession effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities 44

61 Chapter 5: Effect of Number of Swirlers on the Flow Field Generated by a Multiple- Swirler Array As mentioned in Chapter 1, a real single annual gas turbine combustor consists of several swirler/ fuel nozzle assemblies placed side-by-side. However, in the early design stage, it is very difficult to conduct the experiment of a full-sized annual combustor due to the cost of design, the time schedule of design, and the capability of test facility. Using the limited number of swirlers in a simplified geometry to understand the aerodynamics of multi-swirler array will be very helpful for engineer to predict the potential problem in the earlier design stage for a production combustor. So, the aim of this chapter is to discuss the effect of number of swirlers on the aerodynamics performance of a multi-swirler array to understand that how many swirler should be used in the early design stage for the research purpose. 5.1 Test Conditions All of data were taken at the air pressure drop as 4±.1% of 1 atm atmosphere pressure cross the swirler. Air temperature ranged from 65 to 75 o F. The LDV measurements were conducted on a single swirler, and the three-, five-swirler arrays with confinement. The confinement chambers for the three cases were 2D x 2D, 6D x 2D, and 1D x 2D, respectively, to maintain the same confinement ratio. Due to the test condition was controlled by the pressure drop cross swirler, the total air flow rate passed through a multiple-swirler array was unknown. Since those swirlers were not placed very close to each other in the multiple-swirler arrays, so the total air flow rate of the three- and five-swriler arrays should be almost equal to three and five times of the air flow rate passed through a single swirler at the same pressure drop test condition. The coordinate system of a multiple-swirler array is shown as Figure 5.1. The axis of the center swirler is assigned as Z-axis. The X-axis is perpendicular to Z-axis and parallel to the direction of the 45

62 multiple-swirler array arrangement. The origin of this coordinate system is at the center of the center swirler exit plane. The measurements were taken along the X-axis with a.8d spatial resolution for the all three cases, and 25 different axial locations were measured. 5.2 Results and Discussions Figure 5.2 shows the axial velocity and the axial-radial velocity vector contours for the three cases: single swirler, three-swirler array, and five-swirler array. The relative length of vector is used in Figure 5.2. The size and the intensity of individual CTRZ in the two configurations with multiple swirlers are not uniform among the entire flow field. However, in two configurations with multiple swirlers still have a commonly observed characteristic. The intensity of CTRZ from the center swirler is relatively weak, and its size is also larger as compared to the two adjacent swirlers which have strong, compact CTRZs. In the region close to the swirler exit, the flow from the center swirlers for the 3- and 5-swirler configurations behaves similarly to that of the unconfined swirler. And the size and the intensity of each CTRZ among multiple-swirler configurations show a periodically alternating distribution can be concluded. Figures 5.3 (a) and (b) show the axial velocity and radial velocity profiles in Z/D =.12, the proximity of swirler exit. In the both multiple-swirler arrays, the swirling jet issues out from the center swirler with lower axial velocity and higher radial velocity, which indicates that the jet goes out by high expansion angle with respect to the axis of swirler, and the flow goes out along the dome plate from the swirler, as opposite to the flow structure of a confined single swirler. The swirler adjacent to the center swirler features a swirling jet with significantly higher magnitude of axial velocity at the location of ±1.5 and ±2.5, so the swirling jet from the adjacent swirler comes out with a smaller expansion, as 46

63 compared to that from the center swirler. There is a small region with slightly negative axial velocity located between any two adjacent swirlers. Figure 5.4 (a) shows the axial, radial, tangential velocity, and turbulent kinetic energy profiles, respectively, at Z/D =.63. The swirling jet issues out by the center swirler, located at ±.6 in Z/D =.12 (Figure 5.3) have been merged into the swirling jet from its neighbor, located at ±1.5. A similar phenomenon also happens at outer region for the 5-swirler configuration, the jet located at ±3.5 gets merged into the jet located at ±2.5. Due to the higher axial velocity surrounded the adjacent swirler of the center swirler, located at ±2, a high pressure drop is generated along the axis of the adjacent swirler, which results in the formation of a stronger reverse flow. Thus, the CTRZ located at ±2 is more intense and compact, as compared to the center CTRZ from the center swirler. The radial velocity profiles shown in Figure 5.4 (b) display a significant drop in the absolute magnitudes of the radial velocity from Z/D =.12 to Z/D =.63 for the center swirler of the multipleswirler configurations. This indicates that the lateral expansion of the center swirler is slowing down so that the CTRZ are near their maximum width at this location. But the adjacent swirler has a reverse trend on the radial velocity distribution, which is attributed to the reducing of size of CTRZ. The tangential velocity distribution at Z/D =.63 for the three configurations in shown in Figure 5.4 (c). The tangential velocity distribution for the 3-swirler configuration shows the same profile for the all three swirlers. However the slope of the tangential velocity profile for the center swirler is smaller than that for the adjacent swirlers, indicating that the clockwise swirling motion of the center swirler is weaker. For the 5-swirler configuration, the trend of tangential velocity is reversed for the center swirler, whereas, all the rest swirlers have a trend for clockwise rotation. The swirler adjacent to the center 47

64 swirler in the 5-swirler configuration has much stronger gradient in the tangential velocity profile, which indicates the adjacent swirler of the center swirler have stronger swirling flow. Since all swirlers are identical, in the region between any two swirlers, the swirling jets from the swirlers have an opposite flow direction in this co-rotating arrangement. In this case, for the swirlers next to the center swirler, it will entrain more of the swirling jet from the center swirler due to the early dispersing of swirling flow from the center swirler. As a result, for the center swirler, the effect of the swirling jet on the flow field is diminished. Moreover, since the flow from the secondary air passage forms the outer portion of the swirling jet, the effect of the secondary air stream gets reduced first. This is observed in the form of the reduced slope of the tangential velocity for the center swirler in the 3-swirler configuration. For the 5- swirler configuration, the interaction seems to have progressed to an extent for the center swirler that the effect of the secondary passage on the flow field is completely gone and the flow retains the counterclockwise motion mainly from the inner, primary air passage, causing a change in the trend of the tangential velocity for the center swirler. In Figure 5.4 (d), the 5-swirler configuration has markedly higher level of turbulent kinetic energy, as compared to those of the 3-swirler and the single swirler configurations, respectively. Those peaks with high turbulent intensity follow those locations where high speed swirling jets are due to the high velocity gradient and hear shear stress. The turbulent kinetic energy profile shows the mixing and swirling flow interaction happened more vigorous at the border of any two adjacent swirlers in 5-swirler configuration, as compared to that in the other two configurations. This also supports the claim from earlier which indicates that a higher interaction resulted in a reverse tangential velocity trend for the center swirler in the 5-swirler configuration. Furthermore, the turbulent kinetic energy level of the 3- swirler configuration is higher than that of the single swirler configuration. So the more number of 48

65 swirler employed into the system, the more vigorous interaction will be happened between each swirling flow can be concluded. As flow develops to the further downstream, Z/D = 1.77 (Figure 5.5), those compact, intense CTRZs located about ±2 for the 3-swirler and 5-swirler configurations, shown at Z/D =.63 are disappeared. Taking 5-swirler configuration as example, the regions at and ±4 having lower axial velocities show the contraction of their sizes from the radial velocity profiles, and the regions at ±2 having higher axial velocities show the expansion of their sizes. Thus, this means the flow structure is under a smoothing process. Additionally, the 3-swirler configuration starts to show the intention of reverse trend on tangential velocity distribution at the center portion, which has positive and negative tangential velocities at the negative, and the positive X locations, respectively. As flow goes to further downstream, the flow structure becomes more uniform, as evident in Figures 5.6, which show the axial and radial velocity comparisons at Z/D = 3.94, respectively. The axial velocities are seen to be nearly uniform for the different configurations. The radial velocities at this location are very close to zero, indicating that there is no significant expansion or contraction of the flow structure. For the both 3- and 5-swirler configurations, the tangential velocities have also merged to a similar pattern. The rotating direction of the center swirler in the 3-swirler configuration also changes its direction after the further counter-flowing interaction in tangential component afterward, as seen in Figure 5.6 (c). As a result, both multi-swirler configurations exhibit the center region with a counterclockwise rotation and the outer region with a clockwise rotating direction. Therefore, we can conclude that due to the strong interaction between the center swirler and its neighbors, the swirling intensity of the center swirler will be weakened. And the rotating direction of the center swirler will be dominated by the inner, primary air passage rather than the outer, secondary air 49

66 passage which is more influencing in the flow structure of a single swirler. The neighbor of the center swirler behaves like a highly confined single swirler, and the CTRZ of the adjacent swirler is much stronger and compact, features a higher speed swirling jet as well. Thus, the aerodynamic border between the center and the adjacent swirlers will be bias to the adjacent swirler rather than match to the physical border. Due to the higher turbulent level and stronger swirling flow interaction observed in the 5-swirler array, the flow structure generated by a 5-swirler array might be more similar to that in an annular gas turbine combustor. Hence, the 5-swirler array will be utilized in the following sections of this study. 5.3 Conclusions 1. The result shows that the aerodynamics behavior from a multiple-swirler array is very complicated and shows a periodically non-uniform topology of fluid mechanics. The result also suggests the evaluation of the swirling flow generated by a single swirler might not enough to predict the flow structure generated by a multiple-swirler array or that in an annular gas turbine combustor. 2. The swirling flow interaction has a clear impact on the flow structure of a multiple-swirler array, and should be considered while designing an annular combustor. 3. The 3-swirler and 5-swirler arrays both show a large, weak CTRZ from the center swirler which is accompanied with two relatively strong, compact CTRZs from its neighbors. The swirling jet issues out from the center swirler with a large expansion angle and remains attaching along the dome plate, like what observed in the single, unconfined swirler configuration. 4. At the further downstream, due to the tangential momentum cancellation, the center portion of flow field would change the rotation to a counter-clockwise direction which is generated by the 5

67 inner, primary air passage, but the outer portion of flow field still remains as a clockwise rotation from the outer, secondary air passage. 5. The high turbulent intensity region follows the locations where high velocity swirling jets are and the swirling flow merging takes place. 6. More swirlers involved into the system, the higher turbulent intensity is observed because of the stronger swirling flow interaction. Therefore, a 5-swirler configuration is more suitable for engineering to predict the performance of an annular combustor in the design stage, as compared to a 3-swirler configuration and a single swirler configuration. 51

68 Figure 5.1: coordinate system (multi-swirler array) (a) (b) 52

69 (c) Figure 5.2: axial velocity and axial-radial velocity vector contours (no. of swirlers); (a) single swirler, (b) three swirlers, (c) five swirlers 4 Z/D=.12 3 Axial Velocity (m/s) single swirler 3 swirlers 5 swirlers (a) 53

70 4 3 Z/D=.12 2 Radial Velocity (m/s) single swirler 3 swirlers 5 swirlers (b) Figure 5.3: velocity profiles at Z/D=.12 (no. of swirlers), (a) axial velocities, (b) radial velocities 4 Z/D=.63 3 Axial Velocity (m/s) single swirler 3 swirlers 5 swirlers (a) 4 3 Z/D=.63 2 Radial Velocity (m/s) single swirler 3 swirlers 5 swirlers (b) 54

71 12 Z/D=.63 8 Tangential Velocity (m/s) single swirler 3 swirlers 5 swirlers (c) single swirler 3 swirlers 5 swirlers Z/D=.63 Turbulent Kinetic Energy (m 2 /s 2 ) (d) Figure 5.4: velocity and turbulent kinetic energy profiles at Z/D=.63 (no. of swirlers), (a) axial velocities, (b) radial velocities, (c) tangential velocities, (d) turbulent kinetic energy distributions 4 Z/D= Axial Velocity (m/s) single swirler 3 swirlers 5 swirlers (a) 55

72 4 3 Z/D= Radial Velocity (m/s) single swirler 3 swirlers 5 swirlers (b) 12 Z/D= Tangential Velocity (m/s) single swirler 3 swirlers 5 swirlers (c) Figure 5.5: velocity profiles at Z/D=1.77 (no. of swirlers), (a) axial velocities, (b) radial velocities, (c) tangential velocities 4 Z/D= Axial Velocity (m/s) single swirler 3 swirlers 5 swirlers (a) 56

73 4 3 Z/D= Radial Velocity (m/s) single swirler 3 swirlers 5 swirlers (b) 12 Z/D= Tangential Velocity (m/s) single swirler 3 swirlers 5 swirlers (c) Figure 5.6: velocity profiles at Z/D=3.94 (no. of swirlers), (a) axial velocities, (b) radial velocities, (c) tangential velocities 57

74 Chapter 6: Effect of Inter-Swirler Spacing on the Flow Field Generated by a 5- Swirler Array The spacing between any two adjacent swirlers in an annular gas turbine combustor is a very critical design parameter for engineer. The fewer amounts of swirler/ fuel nozzle assemblies used in a gas turbine combustor would help reducing the weight of system and decreasing the cost of hardware, but it would also result in a larger spacing between any two adjacent swirlers as well. The larger interswirler spacing could affect the flame propagation during the initial ignition procedure significantly. The fuel lean combustion is a future trend of operating condition of an aircraft in order to achieve the emission target and to save fuel consumption. Thus, the flame propagation of high-attitude relight would be a critical safety concern, if the possible flame lean-blowout takes place while engine operating. Additionally, the fuel/ air mixing, heat transfer, and emission are greatly influenced by the mean and turbulent properties of the flow structure which might be affected by the spacing between any two adjacent swirler. Therefore, the objective of this chapter is intended to study the inter-swirler spacing effect on the aerodynamics performance of a multiple-swirler array. 6.1 Test Conditions All of data were taken at the air pressure drop as 4±.1% of 1atm atmosphere pressure across the swirler. Air temperature ranged from 65 to 75 o F. Four different inter-swirler spacing values, as 1.75D, 2D, 2.5D, and 2.75D were studied. The descriptions of inter-swirler spacing (S) and end wall distance (D w ) which will be discussed in the next chapter are shown in Figure 6.1. The size of confinement chamber was changed with the different spacing value. The corresponding chambers used are 9D x 2D, 1D x 2D, 12D x 2D, and 13D x 2D, respectively, to keep the distance from the axis of the corner swirler to the chamber wall (end wall distance, D w ) constant among the four cases. The coordinate system in this chapter is identical to the coordinate system described in Chapter 5. 58

75 6.2 Results and Discussions Figure 6.2 shows the axial velocity contour and axial-radial vectors for the 5-swirler arrays with inter-swirler spacing of 1.75D, 2D, 2.5D, and 2.75D, respectively. The relative length of vector is used in Figure 6.2. In this section, the D w is maintained as 1D among all four cases. The case with S = 2D has been reported in the Chapter 5 and shows a periodically non-uniform CTRZ distribution, as a weak strong weak CTRZ distribution from the center to the corner swirlers. When the inter-swirling spacing is reduced to 1.75D, the flow structure retains the same pattern. However, when the inter-swirler spacing is increased to 2.5D, the flow pattern is different and shows a completely reverse trend, as a strong weak strong CTRZ distribution from the center to the corner swirlers. When the inter-swirler spacing is further increased to 2.75D, the flow pattern persists. It alludes to the possibility of a critical S where the change in the flow pattern may occur between S = 2D and 2.5D. In the following discussion, the cases with S = 1.75D and 2D will be called as the Low-S arrays and, the cases with S =2.5D and 2.75D as High-S arrays. Figure 6.3 displays the axial velocity and radial velocity profiles among the four cases at Z/D =.12. In order to enable the comparisons, the X-axis, the velocity profiles are normalized by S to colocate the centers of five swirlers. It s should be noted that the end wall distance (D w ) for all cases are constant, as 1D, thus the distances between the axis of corner swirler to the end wall appear differently for the four cases due to the normalization. The same method is applied to Figures 6.4 to 6.6 as well. Basically, the Low-S array features a wider high radial velocity distribution from the center swirler than that in the High-S array, which means the swirling jet issues out from the center swirler with a large expansion angle than that in the Low-S array, as shown in Figure 6.3(b). On the contrary, the high-s array shows a relatively wider band of high radial velocity profile from the swirling flow adjacent to the center swirling flow. 59

76 It should be also noted that the mean axial velocity for the S=1.75D array is greater than that for the S = 2D array since the chamber for the S=1.75D array is smaller, while the air mass flow rate is maintained constant. This can be observed as higher axial velocity peaks for S = 1.75 D in Figure 6.3(a). In general, mean axial velocity increases with decreasing of the inter-swirler spacing, S. The small reverse flow regions between any two adjacent swirlers are stronger in the cases with S = 1.75D. This could be attributed to the smaller expansion angle from the swirling jet, which causes a stronger reverse flow region between any two adjacent swirlers, as compared to the case with S = 2D. When the high speed jet has higher axial momentum toward downstream, i.e. higher axial velocity, the higher speed jet is always accompanied by a stronger reverse flow for the geometry with sudden expansion. In the case with S = 2.5D, the swirling jet issuing out from the swirler adjacent to the center swirler gets entrained into its both neighbors completely at the proximity of swirler exit. This might be attributed to the dispersing characteristic of this swirling flow almost matches the available space for expansion in this case, thus the swirling jet flows along the dome plate very closely. As the S increased to 2.75D, the available room is over the dispersing characteristic of this swirling flow could fill up, thus a pair of reverse flow regions are established next to the swirler adjacent to the center swirler naturally. The naturally established reverse flow region causes the swirling jet coming out with a slightly smaller expansion angle, as compared to the case with S = 2.5D. The swirling jet does not attach to dome plate anymore but still has a relatively large expansion angle. At Z/D =.63, It is more clear to see that the low-s arrays, including the cases with S = 1.75D and 2D, show the different flow pattern, as compared to the high S arrays, and feature a large, wide CTRZ at the center, accompanied by two compact CTRZs, as seen in Figure 6.4(a). Additionally, the merging of swirling jet can be observed at this location. The radial velocity profiles corresponding to the compact, intense CTRZs show the contraction of CTRZ size, as evident by a reverse radial velocity 6

77 profile. On the other hand, the large, weak CTRZs have the relatively flat radial velocity profiles. It means the CTRZs are close to their maximum widths and without any significant contraction or expansion at this location, as shown in Figure 6.4(b). In the low-s arrays, the compact CTRZ adjacent to the center CTRZ features a sharp tangential (azimuthal) velocity gradient to show the high strength of swirling intensity with a clockwise rotation. However, the center CTRZ shows a weak contour-clockwise spinning motion, as discussed in Chapter 5. On the contrary, in the high-s arrays, the strong center CTRZ features a clockwise rotation and is accompanied by two weak CTRZs having clockwise rotation as well, as seen in Figure 6.4(c). Basically, the locations of turbulent kinetic energy peak follow the locations of axial velocity peak due to the high speed shear layer and the merging of swirling jet, as shown in Figure 6.4(d). It should be also noted that the compact CTRZ features higher level of turbulent kinetic energy than the wide CTRZ in the same configuration. Thus, the stronger swirling flow featuring a compact CTRZ with higher reverse flow, a shaper tangential velocity gradient, a higher level of turbulent kinetic is confirmed. The mean turbulent kinetic energy increases with decreasing the inter-swirler spacing due to the higher magnitude of axial mean velocity and the possible higher level of swirling flow interaction due to the swirlers placed closer. The impact of inter-swirler spacing remains further downstream, the low S array and high S array show two different flow topologies. The high axial velocity regions at Z/D = 1.77 correspond to those high axial velocity peaks in the near field region, as shown in Figure 6.5(a). And those high axial velocity regions display the flow expansion, whereas, the low axial velocity regions show the flow contraction, which indicates the flow structure is under a smoothing process. For the low S array, the tangential velocity profile shows a clear counter-clockwise rotation at the center and the clockwise 61

78 spinning motion at the outer region. For the case with S = 1.75D, the corner swirling flow and its neighbor already merge together as a single swirling flow with a clockwise rotation could be observed. The corner swirling flow and its neighbor have not merged together yet for the case with S = 2D. On the other hand, for the high S array, a sharp tangential velocity gradient is observed from the center swirler flow with a clockwise rotation. And its neighbors feature the very weak swirling flows with a slight counter-clockwise rotation. As the flow goes to further downstream, Z/D = 3.94, the axial velocity and radial velocity profiles still show the similar results from what observed at Z/D = 1.77, as shown in Figure 6.6(a) and Figure 6.6(b), although the magnitudes of axial velocity and radial velocity get reduced by the flow smoothing development. The low S array shows a united trend of tangential velocity profile, as shown in Figure 6.6(c). A clockwise rotation occupies the outer region of flow structure, and the central region of flow structure features a counter-clockwise rotation. This feature has been discussed in Chapter 5 for the case with S = 2D. But the high S array exhibit a little complicated vortex structure for the flow structure. When the S = 2.75D, the outer region is still occupied a clockwise rotation. Because all of five swirlers feature the clockwise rotation generated by the outer, secondary air passage, the downstream flow field dominated by the clockwise rotation is not surprising. The central region of flow structure inherits what has been developed in the previous section, thus shows an inner clockwise rotation surrounded by a counter-clockwise rotation. Starting from the center, the vortex structure for the case with S = 2.75D feature clockwise counter-clockwise clockwise rotations. The vortex structure for the case of S = 2.5D is significantly more complex. Two weak clockwise spinning regions are observed at X/S ±1. This phenomenon could be attributed to the center three swirling flow has not merged well by this location, which could be also evident by the relatively uneven axial velocity profiles among the region from X/S 1 to 1, than that from the other three cases, as 62

79 seen in Figure 6.6(a). Five independent clockwise rotations are observed at those locations corresponding to the locations of swirlers. A more detailed explanation for the flow field associated with the S = 2.5D case is not established at this time. 6.3 Conclusions 1. The result suggests the flow pattern of a 5-swirler array is quite sensitive to the change of interswirler spacing. And the swirling flow interaction is also affected by the inter-swirler spacing change. 2. For the cases with S = 1.75D or 2D (Low-S arrays), the flow pattern shows a weak strong weak CTRZ distribution, staring from the center to the corner swirler. At downstream, the central portion of flow pattern is occupied by a counter-clockwise rotation, and a clockwise rotation dominates the outer region of flow structure at the downstream. 3. When S changed to 2.5D or 2.75D (High-S arrays), the flow pattern exhibits a completely reverse trend. This indicates that the possibility of a critical value of inter-swirler spacing, between 2D and 2.5D, results in the flow pattern change for this 5-swirler array. 63

80 Figure 6.1: descriptions of inter-swirler spacing (S) and end wall distance (D w ) (a) (b) 64

81 (c) (d) Figure 6.2: axial velocity and axial-radial vector contours (spacing effect); (a) S=1.75, (b) S=2, (c) S=2.5, (d) S=

82 4 Z/D=.12 3 Axial Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (a) 4 Z/D=.12 3 Radiall Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (b) Figure 6.3: velocity profiles at Z/D=.12 (spacing effect); (a) axial velocities, (b) radial velocities 4 Z/D=.63 3 Axial Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (a) 66

83 4 Z/D= Radial Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (b) 12 Z/D=.63 8 Tangential Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (c) S=1.75D S=2D S=2.5D S=2.75D Z/D=.63 Turbulent Kinetic Energy (m 2 /S 2 ) X/S (d) Figure 6.4: velocity profiles at Z/D=.63 (spacing effect), (a) axial velocities, (b) radial velocities, (c) tangential velocities, (d) turbulent kinetic energy distributions 67

84 4 Z/D= Axial Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (a) 4 Z/D= Radial Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (b) 12 Z/D= Tangential Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (c) Figure 6.5: velocity profiles at Z/D=1.77 (spacing effect), (a) axial velocities, (b) radial velocities, (c) tangential velocities 68

85 4 Z/D= Axial Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (a) 4 Z/D= Radial Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (b) 1 Z/D=3.94 Tangential Velocity (m/s) S=1.75D S=2D S=2.5D S=2.75D X/S (c) Figure 6.6: velocity profiles at Z/D=3.94 (spacing effect), (a) axial velocities, (b) radial velocities, (c) tangential velocities 69

86 Chapter 7: Effect of Alignment of Swirlers on the Flow Field Generated by a 5- Swirler Array In the Chapter 5, the flow structure shows a periodically non-uniform distribution in a multipleswirler (3- or 5-swirler) array and a weak CTRZ located at the central portion. The aim of this chapter is to investigate the effect of alignment of the center swirler in a 5-swirler array to establish the benchmark for investigating the swirling flow interaction. 7.1 Test Conditions All of data were taken at the air pressure drop as 4±.1% of 1atm atmosphere pressure across the swirler. Air temperature ranged from 65 to 75 o F. The 5-swirler array shown in Chapter 5 will be presented in this chapter and designated as baseline case for reference. A modified configuration has the center swirler placed 1/8D in streamwise direction. The confinement chambers for both cases are identical. The coordinate system of the modified configuration is shown as Figure 7.1. The axis of the center swirler is assigned as Z-axis. The X-axis is perpendicular to Z-axis and parallel to the direction of the multiple-swirler array arrangement. The origin of this coordinate system is at the center of the center swirler exit plane in the baseline case, so the center of the exit of center swirler in the baseline case is designated as (X, Z) = (, ). In order to compare the test result of two configurations with the same coordinate system, the center of the exit of center swirler in the modified configuration is designated as (X, Z) =(, 1/8inch) or (, 3.2mm). 7.2 Results and Discussions All of discussion below will be more focused on the interaction between the central three swirlers, because the major purpose of the corner swirler is providing the boundary condition for the central portion of flow domain where gets concerned about the swirling flow interaction. 7

87 Figure 7.2 shows the axial velocity and axial-radial vector contours for the two configurations. For the modified configuration, since the exit plane location of the center swirler has been shifted downstream, the swirling jet from the adjacent swirler has progressed further downstream before it interacts with the swirling jet from the center swirler. It results in a reverse phenomenon where the swirling jet from the center swirler issues out with a smaller expansion angle, as compared to that in the baseline case. It results in that the CTRZ of the center swirler in the modified configuration is compact and strong, whereas, the CTRZ of the adjacent swirler is weak. The CTRZ of the corner swirler is relatively stronger as compared to its neighbor, so the CTRZ pattern in the modified configuration shows a strong weak strong distribution, from the center to the corner swirler. This is a completely reverse trend of what was observed from the baseline configuration, as shown in Figure 7.2(a). Therefore, a moderate change of the dome plate alignment can result in a completely different topology of fluid mechanics in multiple-swirler array due to the swirling flow interaction change could be drawn. Figures 7.3 plots the axial and radial velocity profiles for the both baseline modified configurations at a location of Z/D =.2. For the modified configuration, it can be seen that the peaks corresponding to the swirling jets of the center and the corner swirlers have slightly higher magnitude as compared to the peak for the swirler adjacent to the center swirler. Those small reverse flow regions between any two adjacent swirlers at Z/D =.2 are observed more noticeable, as compared to those in the baseline case. Additionally, for the baseline case, the axial velocity peak of the center swirler has completely disappeared, which means the swirling jet gets entrained into the adjacent swirling jet completely by this location. It can be observed that the radial velocity peak of the center swirler for the modified configuration remains high, which indicates that the swirling flow from the center swirler keeps expanding at this location. And the magnitudes of the radial velocity peaks for the all swirlers in the modified configuration are similar. On the other hand, because the center swirler in the baseline 5-71

88 swirler configuration expands its swirling jet in very near region of swirler exit, its radial velocity decays quickly. The observation can be achieved by comparing the magnitude of radial velocity from around 2~25 m/s in Z/D =.12 (Figure 5.3(b)) to around 12 m/s in Z/D =.2 (Figure 7.3(b)). Figures 7.4 shows the axial, radial, tangential velocity and turbulent kinetic energy profiles at Z/D =.63, respectively. The flow has developed slightly further downstream and the differences between these two configurations become more obvious in the axial velocity distribution. For the modified configuration, the swirling jet from the swirler adjacent to the center swirler gets completely entrained and the only remaining swirling jets are from the center and the corner swirlers. Therefore, the center swirler has an intense, compact CTRZ and is accompanied by two weaker CTRZs from its neighbors. This is reverse of the strong-weak-strong trend observed from the baseline 5-swirler configuration. For the baseline case, the swirling jet from the center swirler has been entrained completely into the swirling jets from the adjacent swirlers. The radial velocity distribution for the modified configuration shows all of 5 CTRZs still expanding their width slightly and doesn t show any clear contraction of the CTRZ, whereas, the adjacent swirler of the center swirler, at X 2, shows a reverse trend of radial velocity profile in baseline case. In the tangential velocity distribution, the baseline 5-swirler configuration exhibits a reversed rotating direction for the center swirler, as mentioned in Chapter 5. The spinning direction for the center swirler in the modified configuration is no longer reversed on account of the stronger CTRZ of the center swirler. The swirling jets of the swirlers adjacent to the center swirler get entrained, resulting in weak spins, as indicated by the low gradient of the tangential velocity in the modified configuration. The gradients for the center and the corner swirlers remain sharply at this location. However, all of 5 CTRZs in the modified configuration still rotate as clockwise rotating direction generated by the outer, secondary air passage, as opposite to what observed in the baseline configuration. In Figure 7.4(d), the distribution of turbulent kinetic energy for the 72

89 baseline and the modified configurations exhibit a very similar profile with the axial velocity profile. High turbulent kinetic energy is observed where the swirling jets interact with each other. Thus for the baseline configuration, the peak of turbulent kinetic energy is observed near the location of the swirling jet of the swirler adjacent to the center swirler. On the other hand, for the modified configuration, the turbulent kinetic energy peak is observed near the swirling jets of the center and the corner swirlers. The impact of the offset center swirler persists far downstream, as seen in Figures 7.5, which shows the distributions of axial, radial and tangential velocity components for the baseline and the modified 5-swirler configurations at Z/D = It can be seen that the two distinct axial velocity profiles can be observed for the two configurations. In the near field region, the locations of high axial velocities were at the swirlers adjacent to the center swirler for the baseline configurations, and at the center and the corner swrlers for the modified configuration. At Z/D = 1.77, velocity peaks can be observed at the locations corresponding to the high velocity region in the near field regions, although the velocity magnitudes have reduced significantly. For the modified configuration, the radial velocity profile shows the flow expansion at = and the flow contractions at = ±2, which means the overall flow pattern is under a smoothing process. The tangential velocity profile of the baseline configuration follows what has been observed at Z/D =.63. The center swirling flow shows a counterclockwise rotation for the baseline configuration, and it is accompanied by two strong swirling flows which are able to maintain the clockwise rotation. On the contrary, the modified configuration displays a different trend of tangential velocity profile. The center strong swirling flow spins by clockwise direction, accompanied by two weak swirling flows with counter-clockwise rotation. Because the swirling flow adjacent to the center swirler is weaker, so the outer portion of its swirling flow gets entrained into the center or the corner swirling flow. The remaining flow structure from the adjacent swirler of the center swirler would be dominated by the inner, primary air passage. 73

90 As the flow develops further downstream, at Z/D = 3.94, the axial velocity profiles between two configurations are opposite to each other among the central portion of flow field (Figure 7.6(a)), which is attributed to the opposite distribution of swirling flow in the near field region. The values of the radial velocity profiles from two configurations are very close to zero, which indicates no significant expansion or contraction of flow structure, as seen in Figure 7.6(b). However, a significant difference is observed in the distribution of the tangential velocity, as seen in Figure 7.6(c). For the baseline configuration, the vortexes from the corner swirler and its neighbor merge together as a single clockwise rotation occupying the outer region, and the center region shows a counter-clockwise rotation from the center swirling flow. On the other hand, for the modified configuration, the center and corner swirling flwos retain their spinning direction as clockwise rotation, whereas the direction of rotation of the swirlier adjacent to the center swirler is couter-clockwise rotation. Thus, starting from the center to the corner, the vortexes are clockwise counter-clockwise clockwise for the modified configuration. 7.3 Conclusions 1. The result suggests that the overall flow pattern is very sensitive to the streamwise alignment of swirler. 2. As the center swirler displacing in the streamwise direction, it changed the pattern of interaction between the swirlers. It results in the swirling flow from the center swirler will come out with a small expansion angle and will not merge with that of the adjacent swirler at the proximity of swirler exit. The offset, center swirling can maintain its swirling intensity and own a compact, strong CTRZ, accompanied by two weak CTRZs from its neighbors. This is a reverse of strong weak strong trend of that swirling flow intensity, which is observed in the baseline configuration with all of swirlers placed levelly. 74

91 3. The tangential velocity profile will also be changed by the swirling flow interaction change. Thus, the modified configuration with an offset, center swirler shows a reversed tangential velocity profile at the central portion of flow structure, as compared to the baseline comfiguration. 4. Not only the near field region but also the far field region are influenced by the alignment of swirler can be drawn. 75

92 Figure 7.1: coordinate system (multiple-swirler array, modified configuration with 1/8D offset from the center swirler) (a) (b) Figure 7.2: axial velocity and axial-radial vector contours (alignment effect); (a) baseline (b) 1/8D offset (modified) 76

93 4 baseline 1/8D offset Z/D=.2 3 Axial Velocity (m/s) (a) 4 3 baseline 1/8D offset Z/D=.2 2 Radial Velocity (m/s) (b) Figure 7.3: velocity profiles at Z/D=.2 (alignment effect), (a) axial velocities, (b) radial velocities 4 baseline 1/8D offset Z/D=.63 3 Axial Velocity (m/s) (a) 77

94 4 3 baseline 1/8D offset Z/D=.63 2 Radial Velocity (m/s) (b) 1 8 baseline 1/8D offset Z/D=.63 6 Tangential Velocity (m/s) (c) baseline 1/8D offset Z/D=.63 Turbulent Kinetic Energy (m 2 /s 2 ) (d) Figure 7.4: velocity and turbulent kinetic energy profiles at Z/D=.63 (alignment effect), (a) axial velocities, (b) radial velocities (c) tangential velocities (d) turbulent kinetic energy 78

95 4 baseline 1/8D offset Z/D= Axial Velocity (m/s) (a) 4 3 baseline 1/8D offset Z/D= Radial Velocity (m/s) (b) 1 8 baseline 1/8D offset Z/D= Tangential Velocity (m/s) (c) Figure 7.5: velocity profiles at Z/D=1.77 (alignment effect), (a) axial velocities, (b) radial velocities (c) tangential velocities 79

96 4 baseline 1/8D offset Z/D= Axial Velocity (m/s) (a) 4 3 baseline 1/8D offset Z/D= Radial Velocity (m/s) (b) 1 8 baseline 1/8D offset Z/D= Tangential Velocity (m/s) (c) Figure 7.6: velocity profiles at Z/D=3.94 (alignment effect), (a) axial velocities, (b) radial velocities (c) tangential velocities 8

97 Chapter 8: Effects of End Wall Distance and Confinement on the Flow Field Generated by a 5-Swirler Array The end wall distance is the distance between the axis of corner swirler to the wall of test chamber, as described in Figure 6.1. Although it is not a parameter for engineer in the design stage of an annual gas turbine combustor, it is still important for engineer to understand the effect of boundary condition on a simplified multiple-swirler array. The Additional effect of confinement on a 5-swirler array will be discussed in this Chapter as well. Thus, the aim of this section is to establish the benchmark about the effects of end wall distance and confinement on the flow field generated by a 5-swirler array to help engineer to apply a proper boundary condition on a multiple-swirler array setup. 8.1 Test Conditions All of data were taken at the air pressure drop as 4±.1% of 1atm atmosphere pressure across the swirler. Air temperature ranged from 65 to 75 o F. Four different end wall distances (D w ), as.75d, 1D, 1.25D, 2D, and an additional unconfined case were studied. The size of confinement chamber was changed with the end wall distance. The corresponding chambers used for the confined cases are 9.5D x 2D, 1D x 2D, 1.5D x 2D, and 12D x 2D, respectively. The inter-swirler spacing remains as 2D among the four confined cases and the additional unconfined case. The configuration with D w = 1D is designated as the confined reference case for discussing the effect of confinement on a 5-swirler array. The coordinate system in this chapter is identical to the coordinate system described in Chapter 5. 81

98 8.2 Results and Discussions Figure 8.1 shows the axial velocity and axial-radial vector contours for the four confined configurations with the end wall distance (D w ) as.75d, 1D, 1.25D, and 2D, and the additional unconfined configuration, respectively. The relative length of vector is used in Figure 8.1. The case with D w = 1D has been discussed in previous chapters and shows a weak strong weak CTRZ distribution from the center to the corner swirlers. In the investigated range of end wall distance, from D w =.75D to 2D, the topology of fluid mechanics remains similarly. The major difference among these configurations is the flow pattern from the corner swirler due to the boundary condition change. Figure 8.2 and 8.3 displays the axial and radial velocity profiles at Z/D =.12 for the confined and unconfined cases with different end wall distance, respectively. All of cases exhibit a high degree of similarity on the axial and radial velocity profiles among the center three swirlers. The case with D w = 1D has been discussed before and shown to have a relatively large, weak CTRZ at the corner. When the D w is reduced to.75d, a very sharp axial velocity gradient is observed at the corner region to show the existence of strong corner recirculation zone (CRZ). As the D w is.75d, the gap between trailing edge of swirler flare and the chamber wall is only.25d. Thus, after the swirling jet comes out from the swirler with a certain expansion angle, it has to take a sharp turn to change the flowing direction toward downstream along the chamber wall. Therefore, a very strong CRZ exists at the corner naturally. And due to the low degree of expansion of the swirling jet from the corner swirlers, the corner CTRZ which becomes more compact and stronger but is still slightly larger than its neighbor. 82

99 When the D w increases to 1.25D, for the corner swirling flow, a significant drop of axial velocity peak from around 25 m/s in the case with D w = 1D to 12 m/s is observed. Although, the radial velocity peak also drops from around 3 m/s to 2 m/s, it still indicates the expansion angle of the corner swirling jet becomes higher in the case with D w = 1.25D. When the D w further increases to 2D, a flow separation happens at the corner. Thus, the higher axial velocity peaks are observed from the corner swirler again, and the swirling jet from the corner swirler is coming out with a reduced expansion angle instead of flowing along the dome plate. Although the axial velocity profiles from confined and unconfined cases are highly similar, but the corner swirler in the unconfined case features a very high magnitude of radial velocity at the location of swirling jet. This results in a large expansion angle of corner swirling jet which performs similar to what observed in a single unconfined swirling flow (Figure 3.2(d)). Whereas, the center three swirling flows are similar between the confined and unconfined cases, in terms of the axial and radial velocity profiles. As the flow develops further downstream, at Z/D =.63, the four confined case cases still have the similar velocity profiles among the center three swirlers, as seen in Figure 8.4. But the CTRZ adjacent to the corner CTRZ in the case with D w = 2D feature a stronger swirling intensity, i.e. higher tangential velocity gradient, which might be attributed to its accompanied by a relatively weak corner CTRZ due to the flow separation remaining, as shown in Figure 8.4(c). The remaining flow separation in the case with D w = 2D could be observed from the Figure 8.4(a) & (c). Among these four cases, the peaks of turbulent kinetic energy match to the peaks of axial velocity. The only difference is that a small region with slightly higher turbulent kinetic is observed at ±5 for the case with D w = 2D because of the flow separation. 83

100 Because the corner swirling flow in the unconfined case is highly similar to a single unconfined swirling flow, as mentioned before, the air flow issued out by the corner swirler has been almost dispersed away by this location. The three velocity components all shows the flat distributions, and the corresponding turbulent kinetic energy level approaches to zero for the unconfined case, as shown in Figure 8.5. In general, the center three swirling flows are still similar between the confined and unconfined cases, although the two swirling flows adjacent to the center swirling flow incline slightly toward the center swirling flow due to the combination of these three swirling flows in the unconfined case. The other clear difference is the rotating direction the center swirling flow between two cases. For the confined case, the rotating direction of the swirling flow is dominated by a counter-clockwise rotating motion from the inner, primary air passage, due to the outer air flow from the outer, secondary air passage getting entrained into the adjacent swirling flows, as mentioned in Chapter 5. For the unconfined case, although the swirling intensity of center swirling flow is much weaker than that of its neighbor, the center swirling flow still remain a clockwise rotating motion generated by the outer, secondary air passage. This should be attributed to the enough air supplied from the ambient to the swirling flows adjacent to the center swirling flow in the unconfined case, thus the center swirling flow is not as suffered as that in the confined case due to the tangential cancellation. By Z/D = 3.94, the impacts of end wall distance remains. The four cases show the similarity among the central region and the discrepancy at the outer region due to the impact of end wall distance, as seen in Figure 8.6. Although the velocity profiles are not very similar between the confined and unconfined cases at Z/D = 3.94, as shown in Figure 8.7. But the further downstream flow pattern is usually not very critical for engineer, in particular for an unconfined case. 84

101 As a result, it can be inferred that the impact of D w influence significantly on the corner swirling flow, does not alter the fundamental nature of the swirling flow interaction. The center region with three swirlers where engineers might mainly concern does not show a significant change with the change of D w. Then the D w of 1D used for the baseline array can be considered to be a reasonable choice for considering the effect of the other factors for a confined multipleswirler array. Moreover, the confinement effect or the presence of side wall is not the major factor to trigger the periodically non-uniform flow pattern of a 5-swirler array could be concluded, since the flow topologies from the confined or unconfined 5-swirler arrays are generally similar. 8.3 Conclusions 1. The effects of end wall distance and confinement mainly influence the corner swirling flow instead of the center three swirling flows where engineers might mainly concern. 2. Because the end wall distance doesn t show a significant impact on the central flow region, the end wall distance chosen as the half of inter-swirlier spacing could be a reasonable setup for a simplified, lab-scaled multiple-swirler array for considering the effect of the other factors. 85

102 (a) (b) 86

103 (c) (d) 87

104 (e) Figure 8.1: Axial velocity and axial-radial vector contours; (a) Dw=.75D, (b) Dw=1D, (c) Dw=1.25D, (d) Dw=2D, (e) Dw=unconfined 4 Z/D=.12 3 Axial Velocity (m/s) Dw=.75D Dw=1D Dw=1.25D Dw=2D (a) 88

105 4 Z/D=.12 3 Radial Velocity (m/s) Dw=.75D Dw=1D Dw=1.25D Dw=2D (b) Figure 8.2: velocity profiles, Z/D=.12 (end wall distance); (a) axial velocities, (b) radial velocities 4 Z/D=.12 3 Axial Velocity (m/s) confined (Dw=1D) unconfined (a) 5 4 Z/D=.12 3 Radial Velocity (m/s) confined (Dw=1D) unconfined (b) Figure 8.3: velocity profiles, Z/D=.12 (confinement effect); (a) axial velocities, (b) radial velocities 89

106 4 Z/D=.63 3 Axial Velocity (m/s) Dw=.75D Dw=1D Dw=1.25D Dw=2D (a) 5 4 Z/D=.63 3 Radial Velocity (m/s) Dw=.75D Dw=1D Dw=1.25D Dw=2D (b) 2 Z/D= Tangential Velocity (m/s) Dw=.75D Dw=1D Dw=1.25D Dw=2D (c) 9

107 18 16 Dw=.75D Dw=1D Dw=1.25D Dw=2D Z/D=.63 Turbulent Kinetic Energy (m 2 /s 2 ) (d) Figure 8.4: velocity and turbulent kinetic energy profiles, Z/D=.63 (end wall distance); (a) axial velocities, (b) radial velocities, (c) tangential velocities, (d) turbulent kinetic energy distributions 4 Z/D=.63 3 Axial Velocity (m/s) confined (Dw=1D) unconfined (a) 5 4 Z/D=.63 3 Radial Velocity (m/s) confined (Dw=1D) unconfined (b) 91

108 2 Z/D= Tangential Velocity (m/s) confined (Dw=1D) unconfined (c) confined (Dw=1D) unconfined Z/D=.63 Turbulent Kinetic Energy (m 2 /s 2 ) (d) Figure 8.5: velocity and turbulent kinetic energy profiles, Z/D=.63 (confinement effect); (a) axial velocities, (b) radial velocities, (c) tangential velocities, (d) turbulent kinetic energy distributions 4 Z/D= Axial Velocity (m/s) Dw=.75D Dw=1D Dw=1.25D Dw=2D (a) 92

109 5 4 Z/D= Radial Velocity (m/s) Dw=.75D Dw=1D Dw=1.25D Dw=2D (b) 2 Z/D= Tangential Velocity (m/s) Dw=.75D Dw=1D Dw=1.25D Dw=2D (c) Figure 8.6: velocity profiles, Z/D = 3.94 (end wall distance); (a) axial velocities, (b) radial velocities, (c) tangential velocities 4 Z/D= Axial Velocity (m/s) confined (Dw=1D) unconfined (a) 93

110 5 4 Z/D= Radial Velocity (m/s) confined (Dw=1D) unconfined (b) 2 Z/D= Axial Velocity (m/s) confined (Dw=1D) unconfined (c) Figure 8.7: velocity profiles, Z/D = 3.94 (end wall distance); (a) axial velocities, (b) radial velocities, (c) tangential velocities 94

111 Chapter 9: Phenomenological Description of the Periodic Behavior of a 5-swirler Array In this section, a phenomenological description to explain the periodically non-uniform (alternating) CTRZ distribution for the Low-S arrays is presented first and is followed by the explanation for the reversed periodic observed for the High-S arrays and the 5-swirler array with an offset center swirler reported in Chapter Periodic Behavior among 5-swirler Array As discussed earlier, the dome recession distance and the presence of confinement chamber can switch modes of resulting swirling flow pattern generated by a single swirler. The dome-attaching swirling flows (with no confinement and small dome recession distance) can be switched to a wallattaching swirling flow by the presence of confinement chamber or to a free swirling flow by the increase of dome recession distance. For an array of five identical swirlers with high swirl number, spaced uniformly with a small inter-swirler spacing, the classical flow pattern is depicted in Figure 9.1. In this pattern, the swirling flow structures of all swirlers are identical, and small recirculation regions exist between any two adjacent swirlers and the dome plate. The recirculation region provides the required entrained air to the neighboring swirling jet. The 5-swirler arrays reported in the previous chapters are mainly tested with the dome plate nearly aligned with the trailing edge of the swirler flare, i.e. with a very small dome recession distance (D r =.3D). For a single unconfined swirler, the resulting CRZs are strong and do not provide adequate entrained air to the swirling jet. This results in a dome-attaching swirling jet with a large, unbound CTRZ for a single unconfined swirler. In the case of a 5-swirler array, once one of the swirlers switches 95

112 to a dome-attaching swirling jet, the jet itself can provide sufficient entrained air to its neighboring swirlers. Then, the neighboring swirlers can have a small CTRZ wrapped by the free swirling jets, as shown in Figure 9.2. It results in the creation of the periodic behavior observed for a 5-swirler array mentioned before. 9.2 Switching the Periodic Mode As mentioned before, the wall confinement can switch the modes of swirling flow pattern of a single swirler from dome-attaching swirling flow featuring a large CTRZ to wall-attaching swirling flow with a compact CTRZ. The center swirler of a 5-swirler array is farthest away from the end wall and should have a stronger tendency, as compared to the other four swirlers, to become dome-attaching swirling flow with a large CTRZ. Therefore, the adjacent swirler to the center swirler will feature a CTRZ wrapped by the free swirling jets, and the next swirler would have a large CTRZ with dome attaching swirling flow and so on. However, the location of end wall has a clear impact on the corner swirling flow, and the corner CTRZ size is significantly controlled by the confinement effect (the location of end wall). In the odd-number swirler array (3 or 5 swirlers used in this work), the center and corner swirlers are in competition to set the trigger for the modes of periodic behavior as depicted in Figure 9.3. For the case with relatively loose confinement (larger D w /S, the wall distance normalized by the inter-swirler spacing, i.e. smaller inter-swirler spacing), the corner swirler plays a non-dominant role. Then, the center swirler can remain as the dome-attaching swirling jet mode, resulting in the large CTRZs for the center and corner swirlers (Figure 9.3(a)). As the normalized end wall distance reduces (smaller D w /S, i.e. larger inter-swirler spacing), the CTRZ of the corner swirler becomes small. The small corner CTRZ 96

113 triggers the periodic mode to switch from a large small large to a small large small CTRZ distribution from the center to corner swirlers (Figure 9.3 (b)). From the aspect of pressure balance, it can also explain that the flow pattern of a 5-swirler array has to be switched when the normalized end wall distance (D w /S) decreased, i.e. the inter-swirler spacing increased. If the CTRZ distribution remains as large small large from the center to the corner swirler, the flow pattern has to be what described in Figure 9.4 when the D w /S decreases. However, in the case, the corner swirling flow has to be in charge to provide the air stream feeding into the reverse flow region between the corner swirler and its neighbor and acts as a large CTRZ. The corner swirling flow is highly influenced by the presence of confinement. The corner swirling flow is unlikely to be stretched out only along one direction, since there are chamber walls as boundary conditions in the other three directions. When the center swirler is offset along the axial streamwise direction, the swirling flow issued out from the swirler adjacent to the center swirler would provide the air stream to the reverse flow region between the center swirler and its neighbor. Thus, a large CTRZ would be observed from the swirler adjacent to the center swirler. As the result, the center swirler would have a small CTRZ, and the other small CRZ is from the corner swirler, as shown in Figure

114 Figure 9.1: classical flow pattern of an array of five identical swirlers with high swirl number Figure 9.2: sketch of an alternating order of CTRZ distribution (a) 98

115 (b) Figure 9.3: two possible periodic flow patterns of a 5-swirler array: (a) mode triggered by the center swirler (high D w /S); (b) mode triggered by the corner swirler (low D w /S) Figure 9.4: an unlikely flow pattern of a 5-swirler array with low D w /S Figure 9.5: the flow pattern with an offset center swirler 99

116 Chapter 1: Spray Distribution of a 5-Swirler Array After the aerodynamics study of a multiple-swirler array, a following study for the spray distribution is critical for a liquid-fueled combustion system. Since the deposited liquid film on the confinement chamber would block the laser for PDPA measurement, so this work was conducted under the unconfined condition. The unconfined single swirler and the unconfined 5-swirler array were both conducted in order to understand the effect of swirling flow interaction generated by a 5-swirler array on the liquid atomization. The gas phase test results done by LDV measurement in previous chapters would be presented as well in order to compare the difference and the similarity between the gas phase result (LDV data) and the liquid phase result (PDPA data). 1.1 Test Conditions All of data were taken at the air pressure drop as 4±.1% of 1atm atmosphere pressure across the swirler. Air temperature ranged from 65 to 75 o F. The water flow rate for each fuel nozzle was controlled around 4. lb/h and, the total water flow rate among 5 fuel nozzle was 2 lb/h. As mentioned in Chapter 2, the air flow rate for each swirler at 4% of 1atm pressure drop was around 7 lb/h. Thus, the air/water ratio is about which is a reasonable test condition for a gas turbine combustor. The coordinate systems used for the liquid phase (PDPA) measurement are identical to those for the gas phase (LDV) measurement. 1.2 Results and Discussions Figure 1.1 shows the axial velocity contours of single swirler for the gas phase and the liquid phases. And Figure 1.2 and Figure 1.3 show the velocity profiles at Z/D=.12 and Z/D =.51, respectively. The swirling jet of the gas phase issues out along the dome plate and the swirling characteristic disperses away in the proximity of swirler exit quickly, as reported in Chapter 3. The 1

117 overall liquid phase (~1 μm) shows a different flow pattern with the swirling jets penetrating further downstream. Although the liquid phase flow is also recirculated, but the strength of CTRZ of liquid phase is much weaker, as evident by the much lower reverse flow magnitude inside CTRZ than the gas phase. For the liquid phase, the width of CTRZ is also narrower, as compared to that of the gas phase at the proximity of swirler exit, as shown in Figure 1.2(a). From the Figure 1.2(b), the gas phase and the liquid phase both show a similar trend on the radial velocity profile distributions, but the magnitude of radial velocity for the liquid phase is much lower than that from the gas phase, which indicates the smaller dispersing rate of the liquid jet. As the flow develops further downstream, at Z/D =.51, the flow structure of the liquid phase remains a hollow cone structure, but the existence of the CTRZ is already barely observed due to a completely positive axial velocity profile shown in the measurement span, as seen in Figure 1.3(a). And the radial velocity profile shows that the liquid jet expands slowly in the lateral direction at this location, as shown in Figure 1.3(b). After sorting out the liquid phase information to three major groups based on the droplet size, the axial velocity contours from the sorted liquid phases are also shown in Figure 1.1. The expansion angle of the liquid jet of smaller droplets (~2 μm) performs more closely to what observed from the gas phase than the other groups of larger droplets (2~6um or 6~1 μm). The strength of CTRZ is also observed increasing with decreasing the droplet size by evaluating the magnitude of reverse flow at the proximity of swirler exit. For the largest droplet group (6~1 μm), the presence of CTRZ is not very clear. All of droplets coming out from a simplex nozzle should have the positive momentum toward downstream, and some of droplets will start to change their flowing directions due to the momentum exchange with air stream as flow progressed. The small droplet is easily influenced by the air stream, because the inertia of small droplet is much smaller. Thus, the smaller droplet is recirculated back 11

118 toward swirler by the air stream, as compared to the larger droplet, which has higher axial momentum toward the downstream direction inherited from the simplex fuel nozzle. Figure show the volume flux, D 1 (mean diameter), D 32 (sauter mean diameter) contours of a single swirler, respectively. The volume flux contour shows a clear hollow cone spray structure generated by this swirler/ fuel nozzle assembly. The volume flux contour is very similar to the axial velocity contour of largest droplet group (6~1 μm). Since the measurement was conducted under isothermal condition, the density of droplet is almost constant during the measurement. Thus, the volume flux distribution should be almost identical to mass flux distribution, and the larger droplets have more weighting factor on the mass flux distribution, i.e. volume flux distribution. The D 1 and D 32 contours show the droplet size is much smaller in the proximity of swirler exit, and droplet size increases significantly as flow going further downstream. This is attributes to the dispersing effect, which means the smaller droplets are dispersed away to ambient due to the dispersing characteristic of a swirling flow, thus the remaining larger droplets will increase the values of D 1 and D 32. Due to the measurement conducted under an isothermal condition, so the evaporating effect should not have any clear impact on the droplet size distribution. At Z/D =.12, the diameter difference between D1 and D32 is around 25~3 um, but the difference is reduced to around 1~15 um at Z/D =.51, as shown in Figure 1.7. It suggests that the smaller and larger droplets coexist at the proximity of swirler exit. The sauter mean diameter (D 32 ) is a ratio of spray volume to spray surface, thus, the sauter mean diameter is sensitive to the population of large droplet. But the mean diameter (D 1 ) does not favor either small droplet or large droplet. A wider band distribution of liquid size due to the coexistence of small and large droplets will result in a larger difference between D 1 and D 32. Because of the dispersing effect, a large amount of remaining droplet at further downstream is larger, so the 12

119 droplet size distribution changes to a narrow band distribution. It results in a smaller difference between D 1 and D 32 at the further downstream. Figure 1.8 shows the axial velocity contours of a 5-swirler array for the gas phase and the liquid phases. And Figure 1.9 and Figure 1.1 plot the velocity profiles of a 5-swirler array at Z/D=.12 and Z/D =.51, respectively. A periodically non-uniform flow structure of gas phase generated by an unconfined 5-swirler array has been reported in Chapter 8, and the CTRZ distribution is weak strong weak, as starting from the center to the corner swirler. General speaking, the overall liquid phase and three sorted liquid phase all show a similar trend. At Z/D =.12, the velocity profiles from the gas phase already show a clear non-uniform axial and radial velocity distributions, however, the velocity profiles from the liquid phase exhibit the reasonably uniform velocity distributions among all five swirler/ fuel nozzle assemblies, as shown in Figure 1.8 (a) and (b). It also suggests that the liquid jets come out with a relatively similar expansion angle, since the magnitude of peak of radial velocity in the liquid phase is much smaller than that in the gas phase. For the center and two corner swirlers, the air streams issue out along the dome plate, as observed in gas phase result. But, for the liquid phase, the liquid jets comes out detaching the dome plate should be noted. It might be attributed to the inertia of liquid droplet from droplet source still dominates by this location (the proximity of swirler exit) because of the limited amount momentum exchange between air stream and liquid droplet. By comparing the axial velocity contours among three sorted liquid group, the smaller droplets still perform more closely to the gas phase at the proximity of swirler exit could be concluded. In general, the intensity of reverse flow and the level of non-uniform flow distribution increase with decreasing the droplet size because the momentum is much easier to be transported from the gas phase to the smaller droplet than the larger droplet. As the flow processes to further downstream, Z/D =.51, due to the further momentum exchange between air stream and liquid droplet, the velocity profiles from 13

120 the liquid phase exhibit a similar trend, as a weak strong weak CTRZ distribution, starting from the center to the corner swirlers, as seen in Figure 1.1 (a), although the absolute velocity magnitude is much smaller than that in gas phase. By checking the velocity profiles from three sorted liquid groups, the stronger negative velocity inside the CTRZ and higher axial velocity peak from the liquid jet are observed with decreasing the droplet size, as evident in Figure 1.1(c). Thus, the smaller droplets performing more closely to the gas phase because of the smaller droplet is more influenced by air flow could be confirmed in an unconfined 5-swirler array again. Figure show the volume flux, D 1 (mean diameter), D 32 (sauter mean diameter) contours of a 5-swirler array, respectively. Basically, five independent hollow cone spray structures generated by these five swirlers are observed. The droplet size contours show the center three swirlers having smaller droplets than the other two corner swirler. The smaller droplets from the two corner swirlers are more likely dispersed away to ambient, since the two corner swirling flows perform like a single unconfined swirler due to the similar boundary condition. Figure 1.14 shows the D 1 and D 32 profiles of a 5-swirler array at Z/D =.12 and.51. For the center portion of flow domain, the droplet size almost remains the same range as the flow goes further downstream in a 5-swirler array, and the D 1 and D 32 decrease from 3 and 55 to 25 and 5um, respectively. It s opposite to what observed from a single swirler. It could be attributed to two major reasons. Firstly, the dispersing effect in the center portion of 5-swirler array is not as dominant as that in single swirler configuration, thus the smaller droplet is much easily trapped inside the center portion of flow domain due to the bulk flow motion. Secondly, in the 5-swirler array, the additional shear stress provided by the swirling flow interaction acts as a mechanism to break up the droplet to further smaller one. The non-uniform droplet distribution is also evident by checking the axial velocity and volume flux contours at the cross-section plane (XY plane) which is perpendicular to the axis of swirler, as 14

121 shown in Figure The swirler adjacent to the center swirler has higher axial velocity on the ring of hollow cone spray structure and stronger reverse flow inside the CTRZ at Z/D =.51. It also indicates that the swirler adjacent to the swirler has stronger CTRZ and gets the entrained stream from its neighbors. The D 1 and D 32 contours at the cross-section plane are shown in Figure 1.15(c) and Figure 1.15(d), respectively. The droplet size distribution also exhibits that there are larger droplets at the outer region and smaller droplets at the center region due to the dispersing effect, as discussed before. Moreover, the swirler adjacent to the center swirler has smaller D 1 value inside the CTRZ, indicating the stronger CTRZ is able to recirculate more amounts of smaller droplets into it, as compared the CTRZ formed by the center swirler. 1.3 Conclusions 1. For the single unconfined swirler, because the smaller droplet is more influenced by the air flow than the larger droplet, the small droplet performs more closely to what observed in the gas phase. Thus, small droplet will be dispersed away to ambient at the proximity of swirler exit. As a result, the overall droplet size increases as the flow going further downstream and the diameter difference between D 1 and D 32 get reduced at the downstream. 2. For a single swirler case, although the liquid phase also shows a CTRZ, the inertia of larger droplet is hard to be influenced by air stream. A large amount of large droplet will not be recirculated back to the CTRZ by the air stream. The main recirculation region is occupied by the smaller droplet. 3. By the influence of a periodically non-uniform flow distribution of gas phase in an unconfined 5- swirler array, the liquid phase also shows a similar trend of a non-uniform droplet distribution in a 5-swirler array. It suggests that the flow structure from the gas phase has a clear impact on the spray distribution of a 5-swirler array. 15

122 4. In the 5-swirler array, the dispersing effect mainly affects the spray distribution of the corner swirler. The small droplet coming from the center three swirlers are likely trapped inside the central portion of flow domain due to the bulk flow motion. Thus, the droplet size at the central portion of flow structure remains much smaller than the outer region. 5. Due to the swirling flow interaction, the additional shear stress applied on droplet can break up the droplet further smaller, so the droplet size remains the same range as the flow develops further downstream, which is opposite to what observed from a single swirler. 6. According to the similarity between unconfined gas phase and liquid phase of a 5-swirler array, the droplet distribution of a confined 5-swirler array might be inferred as an alternating distribution. 16

123 (a) (b) (c) 17

124 (d) (e) Figure 1.1: axial velocity contours of single swirler configuration: (a) gas phase, (b) liquid phase (~1μm), (c) liquid phase (~2μm), (d) liquid phase (2~6μm), (e) liquid phase (6~1μm) 2 15 gas phase liquid phase (~1um) Z/D=.12 Axial Velocity (m/s) (a) 18

125 4 3 gas phase liquid phase (~1um) Z/D=.12 2 Radial Velocity (m/s) (b) Figure 1.2: velocity profiles of single swirler, Z/D =.12; (a) axial velocities, (b) radial velocities 2 15 gas phase liquid phase (~1um) Z/D=.51 Axial Velocity (m/s) (a) 4 3 gas phase liquid phase (~1um) Z/D=.51 2 Radial Velocity (m/s) (b) Figure 1.3: velocity profiles of single swirler, Z/D =.51; (a) axial velocities, (b) radial velocities 19

126 Figure 1.4: volume flux contour of single swirler Figure 1.5: mean diameter (D 1 ) contour of single swirler Figure 1.6: sauter mean diameter (D 32 ) contour of single swirler 11

127 8 7 D1 (mean diameter) D32 (sauter mean diameter) Z/D=.12 droplet diameter (um) (a) 8 7 D1 (mean diameter) D32 (sauter mean diameter) Z/D=.51 droplet diameter (um) (b) Figure 1.7: D 1 and D 32 comparisons of single swirler; (a) Z/D =.12, (b) Z/D =.51 (a) 111

128 (b) (c) (d) 112

129 (e) Figure 1.8: axial velocity contours of 5-swirler array: (a) gas phase, (b) liquid phase (~1μm), (c) liquid phase (~2μm), (d) liquid phase (2~6μm), (e) liquid phase (6~1μm) 4 Z/D=.12 3 Axial Velocity (m/s) gas phase liquid phase (~1um) (a) 5 4 gas phase liquid phase (~1um) Z/D=.12 3 Radial Velocity (m/s) (b) 113

130 4 Z/D=.12 3 Axial Velocity (m/s) ~2um 2~6um 6~1um (c) Figure 1.9: velocity profiles of 5-swirler array, Z/D =.12; (a) axial velocities (gas v.s. liquid), (b) radial velocities (gas v.s. liquid), (c) axial velocities (sorted liquid phase) 4 gas phase liquid phase (~1um) Z/D=.51 3 Axial Velocity (m/s) (a) 5 4 gas phase liquid phase (~1um) Z/D=.51 3 Radial Velocity (m/s) (b) 114

131 4 ~2um 2~6um 6~1um Z/D=.51 3 Axial Velocity (m/s) (c) Figure 1.1: velocity profiles of 5-swirler array, Z/D =.51; (a) axial velocities (gas v.s. liquid), (b) radial velocities (gas v.s. liquid), (c) axial velocities (sorted liquid phase) Figure 1.11: volume flux contour of 5-swirler array 115

132 Figure 1.12: mean diameter (D 1 ) contour of 5-swirler array Figure 1.13: sauter mean diameter (D 32 ) contour of 5-swirler array 8 7 D1 (mean diameter) D32 (sauter mean diameter) Z/D=.12 droplet diameter (um) (a) 116

133 8 7 D1 (mean diameter) D32 (sauter mean diameter) Z/D=.51 droplet diameter (um) (b) Figure 1.14: D 1 and D 32 comparisons of single swirler; (a) Z/D =.12, (b) Z/D =.51 swirler (a) (b) 117

134 (a) (b) Figure 1.15: spray distribution information of 5-swirler array at XY plane (Z/D =.51); (a) axial velocity contour, (b) volume flux contour, (c) mean diameter (D 1 ) contour, (d) sauter-mean diameter (D 32 ) contour 118

135 Chapter 11: Flame Shape of a 5-Swirler Array In this chapter, the flame shape generated by a confined 5-swirler array with different interswirler spacing will be discussed to examine the qualitative relationship between non-reacting flow and reacting flow. Tw inter-swirler spacings (S), as 2D and 2.5D were conducted, since the flow pattern with S = 2D (large small large CTRZ distribution from the center to corner swirlers) is different to that with S = 2.5D (small large small CTRZ distribution from the center to corner swirlers), as reported in Chapter 6 for non-reacting aerodynamic study Test Conditions All of images were taken two different total fuel flow rate of 14.8 and 16. lb/h with four different air pressure drop settings across the swirler, as 14, 16, 18, and 2 inh 2 O. And the fuel flow rate was uniformly distributed to 5 fuel nozzles and monitored by rotameters and pressure transducers, as mentioned in Chapter 2. The preheated air temperature was maintained around 4±1 o F. Two interswirler spacings, as 2D and 2.5D were tested. Eight different air/ fuel combined test conditions were investigated for each inter-swirler spacing setting. The original video files of flame shape were captured by a high speed camera with a constant recoding speed of 5 frames per second (fps). The time-averaged images of flame shape were processed by the free software ImageJ which is developed at National Institutes of Health. And the time-averaged image of flame shape is averaged by 8 instantaneous images at 5 fps Results and Discussions Figure 11.1 shows the flame shapes generated by the 5-swirler array with S = 2D among eight test conditions. As observed, the flame anchored by the center swirler is more yellowish and sooty, as compared to its neighbor. Thus, the relatively large CTRZs (the CTRZs from the center and corner 119

136 swirlers) found out from the non-reacting aerodynamic (LDV) measurement feature fuel richer flame, referred to Figure 6.2(b). Whereas, the flame anchored by the swirler adjacent to the center swirler always looks like cleaner (much blue), even compared to the flame anchored by the corner swirler. This result should be mainly attributed to the swirling flow adjacent to the center swirling flow gets entrained air from its neighbors as the dilution air to result in a leaner combustion at this region. As a result, the regional equivalence ratio of the flame anchored by the swirling flow adjacent to the center swirler might be leaner than the overall equivalence ration which was simply calculated from the overall air/ fuel ratio. Thus, the regional equivalence ratio associated with the flames anchored by the center and corner swirling flows might be underestimated by the overall equivalence ratio. Moreover, the cleaner flame from the swirling flow adjacent to the center swirler might also be inferred that a more efficient combustion happens due to the smaller fuel droplet (referred to Figure 1.12 and Figure 1.13) and higher turbulent level at this region (referred to Figure 6.4(d)). For the other 5-swirler array with S = 2.5D, the general flame distribution is relatively uniform and doesn t show a much sooty flame at the center portion of flow domain than the outer region, as shown in Figure For the non-reacting flow, when the 5-swirler array is changed from a Low-S array to a High-S array, the flow pattern will be switched and show a small large small CTRZ distribution from the center to the corner swirler, as reported in Chapter 6. As a result, it has been inferred that there is a critical value of inter-swirler spacing between S = 2D and 2.5D to alter the flow pattern. In the reacting case, due to the volume expansion in the process of heat energy released by fuel, the flow structure of reacting case will have much higher axial momentum, as compared to the nonreacting case. Thus, the axial/ tangential momentum ratios should be different between the reacting and non-reacting flows, as also mentioned by several researchers [51, 52, 53]. The swirling flow interaction is highly influent by the tangential/ axial momentum ratio of each swirling flow employed, i.e. strength 12

137 of swirling flow. Thus, the discrepancy of non-reacting flow pattern and reacting flame shape should be attributed to the change of swirling flow property from a non-reacting case to a reacting case. A more detailed explanation for the discrepancy is not established at this time Conclusions 1. For reacting flow in a 5-swirler array with S = 2D, the general flame shape is somehow similar to what observed in the non-reacting flow aerodynamics. The large CTRZ from the center swirler is associated with the sooty region of reacting flow. The flame anchored by the swirling flow adjacent to the center swirling flow in clear and leaner, because the more entrained air is involved into the combustion and the high efficient combustion happens probably. 2. When the inter-swirler spacing is increased to 2.5D, the reacting flow show a relatively uniform flame shape distribution instead of a periodically non-uniform distribution similar to what observed from the non-reacting aerodynamic study. 3. The discrepancy between the non-reacting flow structure and the flame shape of reacting case should be attributed to the change of tangential/ axial momentum from each swirler which results in the change of swirling flow interaction. 121

138 dp = 14 inh2o = 14.8 lb/h ɸ =.85 dp = 16 inh2o = 14.8 lb/h ɸ =.8 dp = 18 inh2o = 14.8 lb/h ɸ =.75 dp = 2 inh2o = 14.8 lb/h ɸ =.71 dp = 14 inh2o = 16. lb/h ɸ =.92 dp = 16 inh2o = 16. lb/h ɸ =.86 dp = 18 inh2o = 16. lb/h ɸ =.81 dp = 2 inh2o = 16. lb/h ɸ =.77 Figure 11.1: flame shapes generated by a confined 5-swirler array with S = 2D among 8 test conditions 122

139 dp = 14 inh2o = 14.8 lb/h ɸ =.85 dp = 16 inh2o = 14.8 lb/h ɸ =.8 dp = 18 inh2o = 14.8 lb/h ɸ =.75 dp = 2 inh2o = 14.8 lb/h ɸ =.71 dp = 14 inh2o = 16. lb/h ɸ =.92 dp = 16 inh2o = 16. lb/h ɸ =.86 dp = 18 inh2o = 16. lb/h ɸ =.81 dp = 2 inh2o = 16. lb/h ɸ =.77 Figure 11.2: flame shapes generated by a confined 5-swirler array with S = 2.5D among 8 test conditions 123